Chapter XV: Nervous System (1)

_Origin of the Nervous System._

One of the most important recent embryological discoveries is the fact that the central nervous system, in all the Metazoa in which it is fully established, is (with a few doubtful exceptions) derived from the primitive epiblast[152]. As we have already seen that the epiblast represents to a large extent the primitive epidermis, the fact of the nervous system being derived from the epiblast implies that the functions of the central nervous system, which were originally taken by the whole skin, became gradually concentrated in a special part of the skin which was step by step removed from the surface, and has finally become in the higher types a well-defined organ imbedded in the subdermal tissues.

[152] Whether there is any part of it in many types not so
derived requires further investigation, now that it has been
shewn by the Hertwigs that part of the system develops from the
endoderm in some Coelenterata. O. Hertwig holds that part of it
has a mesoblastic origin in Sagitta, but his observations on this
point appear to me very inconclusive. It would be very
advantageous to investigate the origin of Auerbach's plexus in
Mammalia.

Before considering in detail the comparative development of the nervous system, it will be convenient shortly to review the present state of our knowledge on the general process of its evolution.

This process may be studied either embryologically, or by a comparison of the various stages in its evolution preserved in living forms. Both the methods have led to important results.

The embryological evidence shews that the ganglion-cells of the central part of the nervous system are originally derived from the simple undifferentiated epithelial cells of the surface of the body, while the central nervous system itself has arisen from the concentration of such cells in special tracts. In the Chordata at any rate the nerves arise as outgrowths of the central organ.

Another important fact shewn by embryology is that the central nervous system, and percipient portions of the organs of special sense, especially of optic organs, are often formed from the same part of the primitive epidermis. Thus the retina of the Vertebrate eye is formed from the two lateral lobes of the primitive fore-brain.

The same is true for the compound eyes of some Crustacea. The supraoesophageal ganglia of these animals are formed in the embryo from two thickened patches of the epiblast of the procephalic lobes. These thickened patches become gradually detached from the surface, remaining covered by a layer of epidermis. They then constitute the supraoesophageal ganglia; but they form not only the ganglia, but also the retinulæ of the eye--the parts in fact which correspond to the rods and cones in our own retina. The accessory parts of these organs of special sense, viz. the crystalline lens of the Vertebrate eye, and the corneal lenses and crystalline cones of the Crustacean eye, are independently formed from the epiblast after the separation of the part which becomes the central nervous system.

In the Acraspedote Medusæ the rudimentary central nervous system has the form of isolated rings, composed of sense-cells prolonged into nervous fibres, surrounding the stalks of tentacle-like organs, at the ends of which are placed the sense-organs.

This close connection between certain organs of special sense and ganglia is probably to be explained by supposing that the two sets of structures actually originated _pari passu_.

We may picture the process as being somewhat as follows:--

It is probable that in simple ancestral organisms the whole body was
sensitive to light, but that with the appearance of pigment-cells in
certain parts of the body, the sensitiveness to light became
localised to the areas where the pigment-cells were present. Since,
however, it was necessary that stimuli received by such organs
should be communicated to other parts of the body, some of the
epidermic cells in the neighbourhood of the pigment-spots, which
were at first only sensitive in the same manner as other cells of
the epidermis, became gradually differentiated into special
nerve-cells. As to the details of this differentiation embryology
does not as yet throw any great light; but from the study of
comparative anatomy there are grounds for thinking that it was
somewhat as follows:--Cells placed on the surface sent protoplasmic
processes of a nervous nature inwards, which came into connection
with nervous processes from similar cells placed in other parts of
the body. The cells with such processes then became removed from the
surface, forming a deeper layer of the epidermis below the sensitive
cells of the organ of vision. With the latter cells they remained
connected by protoplasmic filaments, and thus they came to form a
thickening of the epidermis underneath the organ of vision, the
cells of which received their stimuli from those of the organ of
vision, and transmitted the stimuli so received to other parts of
the body. Such a thickening would obviously be the rudiment of a
central nervous system, and is in fact very similar to the
rudimentary ganglia of the Acraspeda mentioned above. It is easy to
see by what steps it might become larger and more important, and
might gradually travel inwards, remaining connected with the
sense-organ at the surface by protoplasmic filaments, which would
then constitute nerves. The rudimentary eye would at first merely
consist of cells sensitive to light, and of ganglion-cells connected
with them; while at a later period optical structures, constituting
a lens capable of throwing an image of external objects upon it,
would be developed, and so convert the whole structure into a true
organ of vision. It has thus come about that, in the development of
the individual, the retina is often first formed in connection with
the central nervous system, while the lenses of the eye are
independently evolved from the epidermis at a later period.

A series of forms of the Coelenterata and Platyelminthes affords us examples of various stages in the differentiation of a central nervous system[153].

[153] Our knowledge on this subject is especially due to the
brothers Hertwig (Nos. 320 and 321), Eimer (No. 318), Claus (No.
317), Schäfer (No. 326), and Hubrecht (No. 323).

In sea-anemones (Hertwigs, No. 321) there are, for instance, no organs of special sense, and no definite central nervous system. There are, however, scattered throughout the skin, and also throughout the lining of the digestive tract, a number of specially modified epithelial cells, which are no doubt delicate organs of sense. They are provided at their free extremity with a long hair, and are prolonged on their inner side into fine processes which penetrate into the deeper part of the epithelial layer of the skin or digestive wall. They eventually join a fine network of protoplasmic fibres which forms a special layer immediately within the epithelium. The fibres of this network are no doubt essentially nervous. In addition to fibres there are, moreover, present in the network cells of the same character as the multipolar ganglion-cells in the nervous system of Vertebrates, and some of these cells are characterised by sending a process into the superjacent epithelium. Such cells are obviously intermediate between neuro-epithelial cells and ganglion-cells; and it is probable that the nerve-cells are, in fact, sense-cells which have travelled inwards and lost their epithelial character.

In the Craspedote Medusæ (Hertwigs, No. 320) the differentiation of the nervous system is carried somewhat further. There is here a definite double ring, placed at the insertion of the velum, and usually connected with sense-organs. The two parts of the ring belong respectively to the epithelial layers on the upper and lower surfaces of the velum, and are not separated from these layers; they are formed of fine nerve-fibres and ganglion-cells. The epithelium above the nerve rings contains sense-cells (fig. 237) with a stiff hair at their free extremity, and a nervous prolongation at the opposite end, which joins the nerve-fibres of the ring. Between such cells and true ganglion-cells an intermediate type of cell has been found (fig. 237 B) which sends a process upwards amongst the epithelial cells, but does not reach the surface. Such cells, as the Hertwigs have pointed out, are clearly sense-cells partially transformed into ganglion-cells.

A still higher type of nervous system has been met with amongst some primitive Nemertines (Hubrecht, No. 323), consisting of a pair of large cephalic ganglia, and two well-developed lateral ganglionic cords placed close beneath the epidermis. These cords, instead of giving off definite nerves, as in animals with a fully differentiated nervous system, are connected with a continuous subdermal nervous plexus.

[FIG. 236. NEURO-EPITHELIAL SENSE-CELLS OF AURELIA AURITA. (From
Lankester; after Schäfer.)]

The features of the embryology and the anatomy of the nervous system, to which attention has just been called, point to the following general conclusions as to the evolution of the nervous system.

(1) The nervous system of the higher Metazoa appears to have been evolved in the course of a long series of generations from a differentiation of some of the superficial epithelial cells of the body, though it is possible that some parts of the system may have been formed by a differentiation of the alimentary epithelium.

(2) An early feature in the differentiation consisted in the growth of a series of delicate processes of the inner ends of certain epithelial cells, which became at the same time especially differentiated as sense-cells (figs. 236 and 237).

[FIG. 237. ISOLATED CELLS BELONGING TO THE UPPER NERVE-RING OF
CARMARINA HASTATA. (After O. and R. Hertwig.)

A. Neuro-epithelial sense-cell. _c._ sense-hair.
B. Transitional cell between a neuro-epithelial cell and a
ganglion-cell.]

(3) These processes gave rise to a subepithelial nervous plexus, in which ganglion-cells, formed from sense-cells which travelled inwards and lost their epithelial character (fig. 237 B), soon formed an important part.

(4) Local differentiations of the nervous network, which was no doubt distributed over the whole body, took place partly in the formation of organs of special sense, and partly in other ways, and such differentiations gave rise to a central nervous system. The central nervous system was at first continuous with the epidermis, but became separated from it and travelled inwards.

(5) Nerves, such as we find them in the higher types, originated from special differentiations of the nervous network, radiating from the parts of the central nervous system.

The following points amongst others are still very obscure:--

(1) The steps by which the protoplasmic processes from the primitive
epidermic cells became united together so as to form a network
of nerve-fibres, placing the various parts of the body in
nervous communication.
(2) The process by which nerves became connected with muscles, so
that a stimulus received by a nerve-cell could be communicated
to and cause a contraction in a muscle.

It is probable, as stated in the above summary, that the nervous network took its origin from processes of the sense-cells. The processes of the different cells probably first met and then fused together, and, becoming more arborescent, finally gave rise to a complicated network.

[FIG. 238. MYOEPITHELIAL CELLS OF HYDRA. (From Gegenbaur; after
Kleinenberg.)

_m._ contractile fibres; processes of cells.]

The primitive relations between the nervous network and the muscular system are matters of pure speculation. The primitive muscular cells consist of epithelial cells with muscular processes (fig. 238), but the branches of the nervous network have not been traced into connection with the muscles in any Coelenterata except Ctenophora. In the higher types a continuity between nerves and muscles in the form of motorial end plates has been widely observed. Even in the case of the Coelenterata it is quite clear from Romanes' experiments that stimuli received by the nerves are capable of being transmitted to the muscles, and that there must therefore be some connection between nerves and muscles. How did this connection originate?

Epithelial cells with muscular processes (fig. 238) were discovered by Kleinenberg (No. 324) in Hydra before epithelial cells with nervous processes were known, and Kleinenberg pointed out that Hydra shewed the possibility of nervous and muscular tissues existing without a central nervous system, and suggested that the epithelial part of the myoepithelial cells was a sense-organ, and that the connecting part between this and the contractile processes was a rudimentary nerve. He further supposed that in the subsequent evolution of these elements the epithelial part of the cell became a ganglion-cell, while the part connecting this with the muscular tail became prolonged so as to form a true nerve. The discovery of neuro-epithelial cells existing side by side with myoepithelial cells demonstrates that this theory must in part be abandoned, and that some other explanation must be given of the continuity between nerves and muscles. The hypothetical explanation which most obviously suggests itself is that of fusion.

It seems quite possible that many of the epithelial cells of the epidermis and walls of the alimentary tract were originally provided with processes, the protoplasm of which, like that of the Protozoa, carried on the functions of nerves and muscles at the same time, and that these processes united amongst themselves into a network. Such cells would be very similar to Kleinenberg's neuro-muscular cells. By a subsequent differentiation some of the cells forming this network may have become specially contractile, the epithelial parts of the cells ceasing to have a nervous function, and other cells may have lost their contractility and become solely nervous. In this way we should get neuro-epithelial cells and myoepithelial cells both differentiated from the primitive network, and the connection between the two would also be explained. This hypothesis fits in moreover very well with the condition of the neuro-muscular system as we find it in the Coelenterata.

BIBLIOGRAPHY.

_Origin of the Nervous System._

(316) F. M. Balfour. "Address to the Department of Anat. and Physiol. of the British Association." 1880.

(317) C. Claus. "Studien üb. Polypen u. Quallen d. Adria. 1. Acalephen, Discomedusen," _Denk. d. math.-naturwiss. Classe d. k. Akad. Wiss. Wien_, Vol. XXXVIII. 1877.

(318) Th. Eimer. _Zoologische Studien a. Capri. 1. Ueber Beroë ovatus. Ein Beitrag z. Anat. d. Rippenquallen._ Leipzig, 1873.

(319) V. Hensen. "Zur Entwicklung d. Nervensystems." _Virchow's Archiv_, Vol. XXX. 1864.

(320) O. and R. Hertwig. _Das Nervensystem u. d. Sinnesorgane d. Medusen._ Leipzig, 1878.

(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Berücksichtigung d. Nervenmuskelsystem untersucht." _Jenaische Zeit._, Vol. XIII. 1879.

(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." _Jenaische Zeitschrift_, Vol. XIV. 1880.

(323) A. W. Hubrecht. "The Peripheral Nervous System in Palæo- and Schizonemertini, one of the layers of the body-wall." _Quart. J. of Micr. Science_, Vol. XX. 1880.

(324) N. Kleinenberg. _Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung._ Leipzig, 1872.

(325) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." _Mém. Acad. Pétersbourg_, Series VII., Vol. XVI. 1871.

(326) E. A. Schäfer. "Observations on the nervous system of Aurelia aurita." _Phil. Trans._ 1878.

Nervous system of the Invertebrata. Our knowledge of the development of the central nervous system is still very imperfect in the case of many Invertebrate groups. In the Echinodermata and some of the Chætopoda it is never detached from the epidermis, and in such cases its origin is clear without embryological evidence.

In the majority of groups the central nervous system may be reduced to the type of a pair of cephalic ganglia, continued posteriorly into two cords provided with nerve-cells, which may coalesce ventrally or be more or less widely separated, and be unsegmented or segmented. Various additional visceral ganglia may be added, and in different instances parts of the system may be much reduced, or peculiarly modified. The nervous system of the Platyelminthes (when present), of the Rotifera, Brachiopoda, Polyzoa (?), the Mollusca, the Chætopoda, the Discophora, the Gephyrea, the Tracheata, and the Crustacea, the various small Arthropodan phyla (Poecilopoda, Pycnognida, Tardigrada, &c.), the Chætognatha (?), and the Myzostomea, probably belongs to this type.

The nervous system of the Echinodermata cannot be reduced to this form; nor in the present state of our knowledge can that of the Nematelminthes or Enteropneusta.

It is only in the case of members of the former set of groups that any adequate observations have yet been made on the development of the nervous system, and even in the case of these groups observations which have any claim to completeness are confined to certain members of the Chætopoda, the Arthropoda and the Mollusca. An account of imperfect observations on other forms, where such have been made, will be found in the systematic part of this work.

Chætopoda. We are indebted to Kleinenberg (No. 329) for the most detailed account which we have of the development of the central nervous system in the Chætopoda.

[FIG. 239. SECTION THROUGH THE HEAD OF A YOUNG EMBRYO OF LUMBRICUS
TRAPEZOIDES. (After Kleinenberg.)

_c.g._ cephalic ganglion; _cc._ cephalic portion of the body cavity;
_x._ oesophagus.]

The supraoesophageal ganglion with the oesophageal commissure developes independently of the ventral cord. It arises as an unpaired thickening of the epiblast, close to the dorsal side of the oesophagus at the front end of the head (fig. 239), which becomes separated from the epiblast, and extends obliquely backwards and downwards in a somewhat arched form; its lower extremities being somewhat swollen. The inner portion of this curved rudiment becomes converted into commissural nerve-fibres, while the cells of the outer and upper portion assume the characters of ganglion-cells. The commissural fibres are continued downwards to meet the ventral chord, but their junction with the latter structure is not effected till late in embryonic life.

The ventral cord is formed by the coalescence of a pair of linear cords, the development of which takes place from before backwards, so that when their anterior part is well developed their posterior part is hardly differentiated. These cords arise, one on each side of a ventral ciliated furrow, first as a single row of epiblast cells, and subsequently as several rows (fig. 240, _Vg_). While still united to the external epiblast, they extend themselves below the cells lining the ventral furrow, and unite into a single nervous band, which however exhibits its double origin by its bilobed section. Before the two cords unite, the groove between them becomes somewhat deep, but subsequently shallows out and disappears. The nervous band, before separating from the epiblast, exhibits, in correspondence with the mesoblastic segments, alternate swellings and constrictions. The former become the ganglia, and the latter the connecting trunks.

[FIG. 240. SECTION THROUGH PART OF THE VENTRAL WALL OF THE TRUNK OF
AN EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.)

_m._ longitudinal muscles; _so._ somatic mesoblast; _sp._ splanchnic
mesoblast; _hy._ hypoblast; _Vg._ ventral nerve-cord; _vv._ ventral
vessel.]

As soon as the cord becomes free from the epiblast, it becomes surrounded by a sheath, formed of somatic mesoblast. In each of the ganglionic enlargements there next appears on the dorsal surface a pair of areas of punctiform material, the substance of which soon differentiates itself into nerve-fibres. These areas, by uniting from side to side, give rise to the transverse commissures, and also by a linear coalescence to the longitudinal commissures of the cord. The cellular parts of the band surrounding them become converted into a ganglionic covering of the cord.

In each ganglion the cells of this ganglionic investment penetrate as a median septum into the cord. A fissure is next formed, dividing this septum into two; it is subsequently continued for the whole length of the cord.

Arthropoda. In the Tracheata and the Crustacea the development of the ventral cord is in the main similar to that in the Chætopods, while that of the supraoesophageal ganglia is as a rule somewhat more complicated. No such clear evidence of an independent development of these two parts, as in the case of the Chætopods, has as yet been produced.

The most primitive type of nervous system amongst the Tracheata is that of Peripatus, where it consists of large supraoesophageal ganglia, continuous with a pair of widely separated but large ventral cords united posteriorly above the anus. These cords have an investment of ganglion-cells for their whole length, and are imperfectly divided into ganglia corresponding in number with the feet.

[FIG. 241. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS. The
embryo from which the section is taken was somewhat younger than
that of fig. 242.

_sp.m._ splanchnic mesoblast; _s.m._ somatic mesoblast; _mc._ median
section of body cavity; _lc._ lateral section of body cavity; _v.n._
ventral nerve cord; _me._ mesenteron.]

The ventral cords are formed as two separate epiblastic ridges (fig. 241, _v.n_), continued in front into a pair of thickenings of the procephalic lobes, which are at first independent of each other, and from which a large part of the supraoesophageal ganglia takes its origin. After the latter have become separated from the epiblast an invagination of the epiblast covering them grows into each lobe (fig. 242), and becoming constricted from the superficial epiblast, which remains as the epidermis, forms a not unimportant part of the permanent supraoesophageal ganglia.

In the Arachnida the mode of development of the nervous system is essentially the same, and the reader will find a detailed account of it for Spiders in Vol. II. pp. 447-451. The ventral cords are here formed as independent and at first widely separated strands (fig. 243, _vn_), which for a long time remain far apart; they are subsequently divided into ganglia and become united by transverse commissures.

The supraoesophageal ganglia are formed as two independent thickenings of the procephalic lobes (fig. 244), which eventually separate from the superficial skin. There is formed however in each of them a semicircular groove (fig. 244, _gr_) lined by the superficial epiblast, which becomes detached from the skin, and is involuted to form part of the ganglia.

[FIG. 242. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.)

The figure shews the jaws (mandibles), and close to them epiblastic
involutions, which grow into the supraoesophageal ganglia. The
antennæ, oral cavity, and oral papillæ are also shewn.]

[FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA
LABYRINTHICA.

The ventral cords have begun to be formed as thickenings of the
epiblast, and the limbs are established.

_me.s._ mesoblastic somite; _vn._ ventral nerve-cord; _yk._ yolk.]

A similar mode of formation of both the ventral cords and the supraoesophageal ganglia obtains in Insects (fig. 245). The ventral cords are however much less widely separated than in Spiders, and early unite in the median line. In the supraoesophageal ganglia the invaginated epiblast has in Lepidoptera (Hatschek) the form of a pit on the dorsal border of the antennæ.

Hatschek states that there takes place an invagination of a median part of the skin between the two ventral cords, for the details of which I must refer the reader to Vol. II. p. 410. He has made more or less similar statements for the earthworm, but his observations in both instances are open to serious doubt.

[FIG. 244. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF
AGELENA LABYRINTHICA.

_st._ stomadæum; _gr._ section through semicircular groove in
procephalic lobe; _ce.s._ cephalic section of body cavity.]

[FIG. 245. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
HYDROPHILUS. (After Kowalevsky.)

A. Transverse section through an embryo in the region of one of the
stigmata.
B. Transverse section through an older embryo.

_vn._ ventral nerve-cord; _am._ amnion and serous membrane; _me._
mesoblast; _me.s._ somatic mesoblast; _hy._ hypoblast (?); _yk._
yolk-cells (true hypoblast); _st._ stigma of trachea.]

Full details as to the development of the nervous system in the Crustacea are still wanting; a fairly complete account of what is known on the subject is given in Vol. II. pp. 521-2. It appears that the ventral cord may either arise as an unpaired thickening of the epiblast (Isopoda), marked however by a shallow median furrow, or from two cords which eventually coalesce[154]. It is not certain how far the supraoesophageal ganglia are usually in the first instance continuous with the ventral cord. In Astacus, the early stages of which have been elaborately investigated by Reichenbach (No. 331), they are stated to be so; the supraoesophageal ganglia are moreover described by this author as having a somewhat complicated origin. Five elements enter into their composition. There is first formed a pair of pits on the procephalic lobes, which become very deep during the Nauplius stage, and are continuous with a pair of epiblastic ridges which pass round the mouth, and join the ventral cords just described. The walls of the pits are believed to form a part of the embryonic ganglia which gives rise to the retina as well as to the optic ganglia. The ridges form the remainder of the ganglia and the oesophageal commissures; while the fifth element is supplied by a median invagination in front of the mouth, which appears at a much later date than the other parts.

[154] Reichenbach (No. 331) holds that the walls of the groove
between the two strands of the ventral cords become invaginated
and assist in the formation of the ventral cord.

In the Isopoda supraoesophageal ganglia are stated to arise as thickenings of the procephalic lobes, which become eventually detached from the epidermis.

The ventral cord is at first unsegmented, but soon becomes partially divided by a series of constrictions into a number of ganglia, corresponding with the segments. The development of the commissural and ganglionic portions takes place much as in the Chætopoda.

The Gephyrea approach closely the types so far dealt with, but the ventral cord in the Inermia is formed as an unpaired thickening of the epiblast. In Echiurus, as has been shewn by Hatschek in an interesting paper on the larva of this species, published since the appearance of the first volume, there is a pair of ventral cords[155]. In correspondence with a general segmentation of the body, which is subsequently lost, these cords become segmented. The two cords unite in the median line, and Hatschek, in accordance with his general view on this subject, states that their junction is effected by means of a median cord of invaginated epiblast. The segmentation of the cords subsequently becomes lost. The supraoesophageal ganglia arise as an unpaired median thickening of the procephalic lobe. No traces of segmentation in the ventral cord have been observed by Spengel in Bonellia, and the supraoesophageal ganglion is formed in this genus as an unpaired band.

[155] "Ueber Entwicklungsgeschichte d. Echiurus." _Arbeit. a. d.
zool. Instit. Wien_, Vol. III. 1880.

In all the groups above considered the nervous system clearly presents the same type of development with various modifications.

It is formed of two parts, viz. (1) the supraoesophageal ganglia, and (2) the ventral cord.

In the simpler forms, Chætopoda and Gephyrea, the supraoesophageal ganglia are usually stated to be formed as an unpaired thickening at the apex of the præoral lobe, which in most cases becomes subsequently bilobed.

In the Arthropoda the unpaired præoral lobe of the Chætopoda is replaced by the so-called procephalic lobes, which are themselves bilobed; and the supraoesophageal ganglia are formed of two independent halves; further complications in development are also generally found.

There is not as yet sufficient evidence to decide whether the supraoesophageal ganglia were primitively developed continuously with, or independently of, the ventral cords.

The ventral cord appears in the embryo as _two independent unsegmented strands_, although in a few cases (some Crustacea and Gephyrea) these cords, by an abbreviation in development, arise as an unpaired median thickening of the epiblast.

The form of nervous system of the Chætopoda, Arthropoda, and Gephyrea is clearly therefore to be derived, as was first pointed out by Gegenbaur, from a more or less similar type to that now found in the Nemertines; and as suggested in the chapter on larval forms (_vide_ p. 378) may perhaps be derived from the elongation of a circular ring, of which the anterior end has become developed into the supraoesophageal ganglia, the lateral parts into the two lateral strands, while the posterior part persists in some forms in the junction of the ventral cords above the anus (Enopla and Peripatus).

Mollusca. While study of the anatomy of the nervous system of the Mollusca, especially of certain primitive genera (Chiton, Haliotis, Fissurella, &c.) leaves little doubt that it is formed on the same type as that of the groups just spoken of, the development, so far as our imperfect knowledge enables us to make definite statements on the subject, is somewhat abnormal[156].

[156] _Vide_ Vol. II., pp. 273, 274.

In the Gasteropoda and Pteropoda the supraoesophageal ganglia appear most probably to be developed either as paired thickenings of the epiblast of the velar area, or as invaginated pits of the velar area, which become detached from the surface, and then become solid (Hyaleacea and Limax). In either case the supraoesophageal ganglia appear to be developed quite independently of the pedal ganglia. The latter, as might be anticipated, are earlier in their development and more constant than the various visceral ganglia; and, if the views above expressed are correct, are homologous with the ventral cord of the Chætopods and Arthropods. Their actual development is very imperfectly known.

The most precise statements on the subject, viz. those of Bobretzky and Fol, would lead us to suppose that they arise in the mesoblast, but it seems more probable that they are formed as thickenings of the sides of the foot.

In the Cephalopods all the ganglia are stated to be differentiated in the mesoblast (Lankester, Bobretzky).

Hatschek[157] has recently given a detailed description of the development of the supraoesophageal and pedal ganglia of Teredo. He finds that the former ganglia arise as an unpaired thickening of the epiblast in the centre of the velar area, and the latter as an unpaired thickening of the epiblast of the ventral side of the body between the mouth and the anus. The two ganglia would thus seem to be disconnected with each other in their development.

[157] "Ueber Entwicklungsgeschichte von Teredo." _Arbeit. a. d.
zool. Instit. Wien_, Vol. III. 1880.

(327) F. M. Balfour. "Notes on the development of the Araneina." _Quart. J. of Micr. Science_, Vol. XX. 1880.

(328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren." _Jenaische Zeitschrift_, Vol. XI. 1877.

(329) N. Kleinenberg. "The development of the Earthworm, Lumbricus Trapezoides." _Quart. J. of Micr. Science_, Vol. XIX. 1879.

(330) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." _Mém. Acad. Pétersbourg_, Series VIII., Vol. XVI. 1871.

(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Flusskrebses." _Zeit. f. wiss. Zool._, Vol. XXIX. 1877.

THE CENTRAL NERVOUS SYSTEM OF THE VERTEBRATA[158].

[158] For the development of the central nervous system in
Amphioxus and the Tunicata the reader is referred to the chapters
dealing with those two groups.

The formation of the cerebrospinal axis of the Chordata from the medullary plate has already been treated at length (pp. 301-304). Before entering into the consideration of the morphological value of the various parts of this cord, it will be convenient to describe the more important features of its ontogeny. For this purpose the two parts into which the nervous axis becomes at an early period divided, viz. the spinal cord and the brain, may be dealt with separately.

The Spinal Cord, shortly after the closure of the medullary canal, has, in all the true Vertebrata, the form of an oval tube; the walls of which are of a fairly uniform thickness, and are composed of several rows of elongated cells. This cord, as development proceeds, usually becomes vertically prolonged in transverse section, and the central canal which it contains also becomes vertically elongated. The variations in shape of the spinal canal are very great at different periods and in different parts of the body, and an attempt to chronicle them would appear, in the present state of our knowledge, to be quite valueless[159]. Fig. 117, in which the spinal cord of the chick of the third day is shewn in transverse section, illustrates the character of the cord at the stage just described. Up to this time the walls of the spinal canal have exhibited an uniform structure. A series of changes now however takes place, which results in the differentiation (1) of the epithelium of the central canal, (2) of the grey matter of the cord, and (3) of the external coating of white matter.

[159] Löwe (No. 341) holds that at an early stage of development
three regions can always be distinguished in any section of the
central canal, viz. (1) a ventral narrow slit, (2) a median
enlargement, and (3) a dorsal slit. Such a form can no doubt
often be observed, but my own observations do not lead me to
attach any special importance to it.

The relative time at which each of these parts becomes developed is not constant in the different forms.

[FIG. 246. SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS' CHICK.

_pcw._ dorsal white column; _lcw._ lateral white column; _acw._
ventral white column; _c._ dorsal tissue filling up the part where
the dorsal fissure will be formed; _pc._ dorsal grey cornu; _ac._
anterior grey cornu; _ep._ epithelial cells; _agc._ anterior
commissure; _pf._ dorsal part of spinal canal; _spc._ ventral part
of spinal canal; _af._ anterior fissure.]

The white matter is apparently the result of a differentiation of the outermost parts of the superficial cells of the cord into longitudinal nerve-fibres, which remain for a long period without a medullary sheath. These fibres appear in transverse sections as small dots. The white matter forms a transparent investment of the grey matter and would seem to contain neither nuclei nor cells[160]. The white matter may from the first form only two masses, one on each side, forming a layer on the _ventral and lateral_ parts of the spinal cord but not extending to the dorsal surface (Elasmobranchii, fig. 185, _W_); or it may form four patches, viz. an anterior and a posterior white column on each side, which lie on a level with the origin of the anterior and posterior nerve-roots (the Fowl, Human embryo, etc.). In whichever of these forms the white matter appears, it is always, at first, a layer of extreme tenuity, which rapidly increases in thickness in the subsequent stages, and extends so as gradually to cover the whole cord (fig. 246).

[160] This holds true at first for Elasmobranchii, but at a later
stage there are present numerous nerve-cells in the white matter,
so that the distinction between the white and grey matter becomes
much less marked than in higher types; in this respect
Elasmobranchii present an approximation to Amphioxus.

The anterior white commissure is formed very shortly after the first appearance of the white matter. The grey matter and the central epithelium are formed by a differentiation of the main mass of the spinal cord. The outer cells lose their epithelial-like arrangement, and, becoming prolonged into fibres, give rise to the grey matter, while the innermost cells retain their primitive arrangement, and constitute the epithelium of the canal. The process of formation of the grey matter would appear to proceed from without inwards, so that some of the cells, which have, on the formation of the grey matter, an epithelial-like arrangement, subsequently become converted into true nerve-cells.

As has already been mentioned, the central epithelium of the nervous system probably corresponds with the so-called epidermic layer of the epiblast.

The grey matter soon becomes prolonged dorsally and ventrally into the posterior and anterior horns. Its fibres may especially be traced in two directions:--(1) round the anterior end of the spinal canal, immediately outside its epithelium and so to the grey matter on the opposite side, forming in this way an anterior grey commissure, through which a decussation of the fibres from the opposite sides is effected: (2) dorsalwards along the outside of the lateral walls of the canal.

There is at this period no trace of the ventral or dorsal fissure, and the shape of the central canal is not very different to what it was at an earlier period. This condition of the spinal cord is especially instructive, as it is very nearly that which is permanent in Amphioxus.

The next event of importance is the formation of the ventral or anterior fissure. This owes its origin to a downgrowth of the anterior horns of the cord on each side of the middle line. The two downgrowths enclose between them a somewhat linear space--the anterior fissure--which increases in depth in the succeeding stages (fig. 246, _af_).

The dorsal or posterior fissure is formed at a later period than the anterior, and accompanies the atrophy of the dorsal section of the embryonically large canal of the spinal cord.

The exact mode of its formation appears to me to be still involved in some obscurity.

In the _Elements of Embryology_ the development of the posterior fissure was described in the following way:

"On the seventh day the most important event is the formation of the
_posterior fissure_.

"This is brought about by the absorption of the roof of the
posterior of the two parts into which the neural canal has become
divided.

"Between the posterior horns of the cord, the epithelium forming the
roof of the, so to speak, posterior canal is along the middle line
covered neither by grey nor by white matter, and on the seventh day
is partially absorbed, thus transforming the canal into a
wedge-shaped fissure, whose mouth however is seen in section to be
partially closed by a triangular clump of elongated cells (fig. 246,
_c_). Below this mass of cells the fissure is open. It is separated
from the 'true spinal canal' by a very narrow space along which the
side walls have coalesced. In the lumbar and sacral regions the two
still communicate.

"We thus find, as was first pointed out by Lockhart Clarke, that the
anterior and posterior fissures of the spinal cord are,
morphologically speaking, entirely different. The anterior fissure
is merely the space left between two lateral downward growths of the
cord, while the posterior fissure is part of the original neural
canal separated from the rest of the cavity (which goes to form the
true spinal canal) by a median coalescence of the side walls."

I confess that I have some doubts as to the complete accuracy of the above statement.

Kölliker gives a full account of the gradual atrophy of the central canal; but I do not fully understand his statements with reference to the formation of the posterior fissure, which in fact appears to be only incidentally mentioned. It would seem from his account that a shallow and somewhat wide dorsal fissure is formed to start with, in the human embryo, by two projections of the posterior white horns. On the atrophy of the central canal this furrow becomes narrowed, but Kölliker does not definitely state how it becomes deepened so as to give rise to the permanent dorsal fissure.

It seems to me probable, though further investigations on the point are still required, that the dorsal fissure is a direct result of the atrophy of the dorsal part of the central canal of the spinal cord.

The walls of the canal coalesce dorsally, and the coalescence gradually extends ventralwards, so as finally to reduce the central canal to a minute tube, formed of the ventral part of the original canal. The epithelial wall formed by the coalesced walls on the dorsal side of the canal is gradually absorbed.

The epithelium of the central canal, at the period when its atrophy commences, is not covered dorsally either by grey or white matter, so that, with the gradual reduction of the dorsal part of the canal, and the absorption of the epithelial wall formed by the fusion of its two sides, a fissure between the two halves of the spinal cord becomes formed. This fissure is the posterior or dorsal fissure. In the process of its formation the white matter of the dorsal horns becomes prolonged so as to line its walls; and shortly after its formation the dorsal grey commissure makes its appearance, which is not improbably derived from part of the epithelium of the original central canal.

_Development of the Brain._

The brain is formed from the anterior portion of the medullary plate. When the medullary plate first becomes differentiated it is not possible to distinguish between the region of the brain and that of the spinal cord. The brain region is however usually very early indicated by a widening of the medullary plate, but does not become sharply marked off from the region of the spinal cord. In many Ichthyopsida (Elasmobranchii (fig. 28, C) and Amphibia (fig. 77, A)) the anterior dilatation gives to the medullary plate, before its sides meet to form a canal, a spatula-like form; which is either not present or less marked in Reptilia, Aves and Mammalia.

The length of the brain as compared to the spinal cord is always very great in the embryo, and in the earliest developmental periods the disproportion in the size of the brain is specially marked, owing to the full number of the somites of the trunk not having been formed. In Elasmobranchii the brain is about one-third of the whole length of the embryo at the stage immediately following the closure of the medullary canal.

The first differentiation of the brain into distinct parts is a very early occurrence, and may take place before (Mammalia) or during the closure of the medullary folds. The brain first becomes divided into two successive lobes or vesicles by a single transverse constriction, and subsequently the posterior of these again becomes divided into two, so that three lobes are formed--known as the fore- the mid- and the hind-brain; of these the hind-brain is usually the longest. In some instances a bilobed stage can hardly be recognised. This primitive division of the brain is shewn in many of the figures already given. The reader may perhaps best refer to fig. 108. On the closure of the medullary groove the lumen of the medullary canal is continued uninterruptedly through the brain, but dilates considerably in each of the cerebral vesicles.

The anterior lobe of the brain becomes converted into the cerebral hemispheres, the thalamencephalon, the primary optic vesicles, and the parts connected with them. The middle lobe becomes the optic lobes (corpora bigemina or corpora quadrigemina in Mammalia) and the crura cerebri; while the posterior lobe becomes converted into the cerebellum and medulla oblongata.

Before describing in detail the changes by which the primary vesicles of the brain become converted into the above parts, it will be convenient to say a few words about the general development of the brain.

The most striking peculiarity with reference to the general development of the brain is a curvature which appears in its axis, known as the cranial flexure. The flexure takes place through the mid-brain, and causes the fore-brain to be gradually bent downwards so that the axis of its floor forms, first, a right angle with that of the hinder part of the brain, and subsequently, as a rule, an acute angle.

[FIG. 247. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG
PRISTIURUS EMBRYO.

_cer._ commencement of the cerebral hemisphere; _pn._ pineal gland;
_In._ infundibulum; _pt._ ingrowth from mouth to form the pituitary
body; _mb._ mid-brain; _cb._ cerebellum; _ch._ notochord; _al._
alimentary tract; _Iaa._ artery of mandibular arch.]

During these changes the brain, in most Amniota at any rate, becomes in the first instance retort-shaped, the cerebral vesicle forming the swollen part of the retort, but subsequently the retort-shape is lost owing to the great development of the vesicle of the mid-brain, which forms the termination of the long axis of the embryo. Figs. 29, 76, and 118, are representative figures of embryos of various vertebrate forms at a period when the mid-brain forms the termination of the long axis of the body.

It is generally stated that the cranial flexure is at its maximum at the stage represented in these figures, and there can be no doubt that viewed from the exterior the cranial flexure ceases to be so marked a feature, and finally disappears as the embryo gradually grows older; but though the mid-brain ceases to form the termination of the long axis of the embryo, the flexure of the brain becomes in many forms absolutely more marked; while in other forms, though stated to diminish, it does not entirely vanish.

[FIG. 248. LONGITUDINAL SECTION THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF DEVELOPMENT.

_cer._ cerebral hemisphere; _pn._ pineal gland; _op.th._ optic
thalamus, connected with its fellow by a commissure (the middle
commissure). In front of it is seen a fold of the roof of the
fore-brain, which is connected with the choroid plexus of the third
ventricle; _op._ optic chiasma; _pt._ pituitary body; _in._
infundibulum; _cb._ cerebellum; _au.v._ passage leading from the
auditory vesicle to the exterior; _mel._ medulla oblongata; _c.in._
internal carotid artery.]

The general nature of the changes which take place will perhaps best be understood by a comparison of figs. 247 and 248 representing longitudinal sections at two stages through the brain of an embryo Elasmobranch. The actual cranial flexure, _i.e._ flexure of the floor of the brain, is obviously greater in the older of the two brains, though viewed from the exterior the axis of this brain appears to be quite straight. In the younger stage, fig. 247, the mid-brain (_mb_) forms the end of the long axis of the body, while in the older one the cerebral hemispheres (_cer_) have grown very greatly, especially forwards and dorsalwards. They have thus come to lie in front of the mid-brain, and to form the end of the long axis of the body, and have at the same time compressed the originally large thalamencephalon against the mid-brain. The same general features may be seen in fig. 250 representing a longitudinal section of the brain of an embryo fowl, and fig. 255 representing a longitudinal section of the brain of a Mammal.

The infundibulum or perhaps rather the point of origin of the optic nerves is to be regarded as the anterior termination of the axis of the base of the brain.

The cranial flexure is least marked in Cyclostomata (fig. 253), Teleostei, Ganoidei, and Amphibia, while it is very pronounced in Elasmobranchii, Reptilia, Aves, and Mammalia. In Teleostei, and still more in Cyclostomata, it permanently remains slight, owing to the small development of the cerebral hemispheres.

In addition to the cranial flexures, two other flexures make their appearance in the base of the brain. A posterior at the junction of the brain and spinal cord, and an anterior at the boundary between the cerebellum and medulla oblongata, just at the point where the pons Varolii is formed in Mammalia. The anterior of these is the most marked and constant; it is shewn in fig. 250. It arises considerably later than the main cranial flexure, and since it is turned the opposite way it assists to a considerable extent in causing the apparent straightening of the cranial axis.

Histogenetic changes[161]. The walls of the brain are at first very thin and, like those of the spinal cord, are formed of a number of ranges of spindle-shaped cells. The processes of each of these cells are stated to be continued through the whole thickness of the wall. In the floor of the hind- and mid-brain a superficial layer of delicate nerve-fibres is formed at an early period. This layer appears in the first instance on the floor and sides of the hind-brain, and very slightly, if at all, later on the floor and the sides of the mid-brain. The cells internal to the nerve-fibres become differentiated into an innermost epithelial layer lining the cavities of the ventricles, and an outer layer of grey matter.

[161] It is not within the scope of this work to give an account
of the histogenesis of the brain; in the statement in the text
only a few points, of some morphological importance, are touched
on.

The similarity of the primitive arrangement and histological character of the parts of the brain behind the cerebral hemispheres to that of the spinal cord is very conclusively shewn by the examination of any good series of sections. In both brain and spinal cord the white matter forms a cap on the ventral and lateral parts considerably before it extends to the dorsal surface. In the medulla the white matter does not eventually extend to the roof owing to the peculiar degeneration which that part undergoes.

In the case of the fore-brain the earliest histological changes, except possibly in Mammals, take place on the same general plan as those of the remainder of the central nervous system[162]; but though the general plan is the same, yet the early histological distinction between the fore-brain, and the mid- and hind-brain is more marked than the distinction between the latter and the spinal cord.

[162] I have worked out these changes in Elasmobranchii, Amphibia
(Salamandra) and Aves.

On the floor and sides of the thalamencephalon, and apparently the whole of the hemispheres of the lower types, there is formed, somewhat later than in the remainder of the brain, a very delicate layer of white matter. The inner part of the wall, which still remains comparatively thin, is not at first clearly divided into an epithelial and nervous layer. This distinction soon however becomes more or less apparent, though it is not so marked as in most other parts of the brain; and it appears that in the subsequent growth the greater part of the original epithelial layer becomes converted into nervous tissue.

In Mammals the same plan of differentiation would seem to be followed, though somewhat less obviously than in the lower types. The walls of the hemispheres become first divided (Kölliker) into a superficial thinner layer of rounded elements, and a deeper and thicker epithelial layer, and between these the fibres of the crura cerebri soon interpose themselves. At a slightly later period a thin superficial layer of white matter, homologous with that of the remainder of the brain, becomes established.

The inner layer, together with the fibres from the crura cerebri, gives rise to the major part of the white matter of the hemispheres and to the epithelium lining the lateral ventricles.

The outer layer of rounded cells becomes divided into (1) a superficial part with comparatively few cells, which, together with its coating of white matter, forms the cortical part of the grey matter, and (2) a deeper layer with numerous cells which forms the main mass of the grey matter of the hemispheres.

The development of the several parts of the brain will now be described.

The hind-brain. The hind-brain is at first an elongated, funnel-shaped tube, the walls of which are of a nearly uniform thickness, though the roof and floor are somewhat thinner than the sides. It forms a direct continuation of the spinal cord, into which it passes without any sharp line of demarcation. The ventricle it contains is known as the fourth ventricle.

The sides become in the chick marked by a series of transverse constrictions, dividing it into lobes, which are somewhat indefinite in number. The first of these remains permanent, and its roof gives rise to the cerebellum. It is uncertain whether the other constrictions have any morphological significance. More or less similar constrictions are present in Teleostei. In Elasmobranchii the medulla presents on its inner face at a late period a series of lobes corresponding with the roots of the vagus and glossopharyngeal nerves, and it is possible that the earlier constrictions may potentially correspond to so many nerve-roots.

[FIG. 249. SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF
THE THIRD DAY OF INCUBATION.]

_IV._ Fourth ventricle. The section shews the very thin roof and
thicker sides of the ventricle. _Ch._ Notochord; _CV._ Anterior
cardinal vein; _CC._ Involuted auditory vesicle; _CC_ points to the
end which will form the cochlear canal; _RL_. Recessus labyrinthi
(remains of passage connecting the vesicle with the exterior); _hy._
Hypoblast lining the alimentary canal; _AO._, _AOA._ Aorta, and
aortic arch.]

Throughout the Vertebrata an anterior lobe of the hind-brain becomes very early marked off, so that the primitive hind-brain becomes divided into two regions which may be conveniently spoken of as the cerebellum (figs. 247 and 248, _cb_) and medulla oblongata. The floor of these regions is quite continuous and is also prolonged without any break into the floor of the mid-brain.

The posterior section of the hind-brain, which forms the medulla, undergoes changes of a somewhat complicated character. In the first place its roof becomes in front very much extended and thinned out. At the raphe, where the two lateral halves of the brain originally united, a separation, as it were, takes place, and the two sides of the brain become pushed apart, remaining united by only a very thin layer of nervous matter, consisting of a single row of flattened cells (fig. 249). As a result of this peculiar growth in the brain, the roots of the nerves of the two sides, which were originally in contact at the dorsal summit of the brain, become carried away from one another, and appear to arise at the sides of the brain.

The thin roof of the fourth ventricle is triangular, or, in Mammalia, somewhat rhomboidal in shape. The apex of the triangle is directed backwards.