Chapter XI: Introduction (2)

The general appearance of the embryo between the 30th and 40th hours of incubation is shewn in fig. 108 from the upper surface, and in fig. 109 from the lower. The outlines of the embryo are far bolder than during the earlier stages. Fig. 109 shews the nature of the folding, by which the embryo is constricted off from the yolk-sack. The folds are complicated by the fact that the mesoblast has already become split into two layers--a splanchnic layer adjoining the hypoblast and a somatic layer adjoining the epiblast--and that the body cavity between these two layers has already become pretty wide in the lateral parts of the body of the embryo and the area pellucida. The fold by which the embryo is constricted off from the yolk-sack is in consequence a double one, formed of two limbs or laminæ, an inner limb constituted by the splanchnopleure, and an outer limb by the somatopleure. The relation of these two limbs is shewn in the diagrammatic longitudinal section (fig. 105), and in the surface view (fig. 109) the splanchnic limb being shewn at _sf_ and the somatic at _so_. Between the two limbs, and closely adjoining the splanchnopleure, is seen the heart (_ht_). At the stage figured the head is well marked off from the trunk, but the first separation between the two regions was effected at an earlier period, on the appearance of the foremost somite (fig. 107). Very shortly after the cephalic region is established, and before the closure of the medullary folds, the anterior part of the neural canal becomes enlarged to form the first cerebral vesicle, from which two lateral diverticula--rudiments of the optic lobes--are almost at once given off (fig. 108, _op.v_). By the stage figured the cephalic part of the neural canal has become distinctly differentiated into a fore- (_f.b_), a mid- (_m.b_) and a hind-brain (_h.b_); and the hind-brain is often subdivided into successive lobes. In the region of the hind-brain two shallow epiblastic invaginations form the rudiments of the auditory pits (_au.p_).

[FIG. 107. DORSAL VIEW OF THE HARDENED BLASTODERM OF A CHICK WITH
FIVE MESOBLASTIC SOMITES. THE MEDULLARY FOLDS HAVE MET FOR PART OF
THEIR EXTENT, BUT HAVE NOT UNITED.

_a.pr._ anterior part of the primitive streak; _p.pr._ posterior
part of the primitive streak.]

A section through the posterior part of the head of an embryo of 30 hours is represented in fig. 110. The enlarged part of the neural tube, forming the hind-brain, is shewn at (_hb_). It is still connected with the epidermis, and at its dorsal border an outgrowth on each side forming the root of the vagus nerve is present (_vg_). The notochord (_ch_) is seen below the brain, and below this again the crescentic foregut (_al_). The commencing heart (_ht_), formed at this stage of two distinct tubes, is attached to the ventral side of the foregut.

On the dorsal side of the foregut immediately below the notochord is seen a small body (_x_) formed as a thickening of the hypoblast. This may possibly be a rudiment of the subnotochordal rod of the Ichthyopsida.

[FIG. 108. EMBRYO OF THE CHICK BETWEEN 30 AND 36 HOURS VIEWED FROM
ABOVE AS AN OPAQUE OBJECT. (Chromic acid preparation.)

_f.b._ front-brain; _m.b._ mid-brain; _h.b._ hind-brain; _op.v._
optic vesicle; _au.p._ auditory pit; _o.f._ vitelline vein; _p.v._
mesoblastic somite; _m.f._ line of junction of the medullary folds
above the medullary canal; _s.r._ sinus rhomboidalis; _t._
tail-fold; _p.r._ remains of primitive groove (not satisfactorily
represented); _a.p._ area pellucida.

The line to the side between _p.v._ and _m.f._ represents the true
length of the embryo.

The fiddle-shaped outline indicates the margin of the pellucid area.
The head, which reaches as far back as _o.f._, is distinctly marked
off; but neither the somatopleuric nor splanchnopleuric folds are
shewn in the figure; the latter diverge at the level of _o.f._, the
former considerably nearer the front, somewhere between the lines
_m.b._ and _h.b._ The optic vesicles _op.v._ are seen bulging out
beneath the superficial epiblast. The heart lying underneath the
opaque body cannot be seen. The tail-fold _t._ is just indicated; no
distinct lateral folds are as yet visible in the region midway
between head and tail. At _m.f._ the line of junction between the
medullary folds is still visible, being lost forwards over the
cerebral vesicles, while behind may be seen the remains of the sinus
rhomboidalis, _s.r._]

In the trunk (fig. 108) the chief point to be noticed is the complete closure of the neural canal, though in the posterior part, where the open sinus rhomboidalis was situated at an earlier stage, there may still be seen a dilatation of the canal (fig. 108, _s.r_), on each side of which are the tail swellings; while the mesoblastic somites stop short somewhat in front of it. Underneath the neural canal may be seen the notochord (fig. 109, _ch_) extending into the head, as far as the base of the mid-brain. At the sides of the trunk are seen the mesoblastic somites (_p.v_), the outer edges of which mark the boundary between the vertebral and lateral plates. A fainter line can be seen marking off the part of the lateral plates which will become part of the body-wall, from that which pertains to the yolk-sack.

[FIG. 109. AN EMBRYO CHICK OF ABOUT THIRTY-SIX HOURS VIEWED FROM
BELOW AS A TRANSPARENT OBJECT.

_FB._ the fore-brain or first cerebral vesicle, projecting from the sides of which are seen the optic vesicles _op_. A definite head is now constituted, the backward limit of the somatopleure fold being indicated by the faint line _S.O._ Around the head are seen the two limbs of the amniotic head-fold: one, the true amnion _a_, closely enveloping the head, the other, the false amnion _a´_, at some distance from it. The head is seen to project beyond the anterior limit of the pellucid area.

The splanchnopleure fold extends as far back as _sp_. Along its
diverging limbs are seen the conspicuous venous roots of the
vitelline veins, uniting to form the heart _h_, already established
by the coalescence of two lateral halves which, continuing forward
as the bulbus arteriosus _b.a_, is lost in the substance of the head
just in front of the somatopleure fold.

_HB._ hind-brain; _MB._ mid-brain; _p.v._ and _v.pl._ mesoblastic
somites; _ch._ front end of notochord; _mc._ posterior part of
notochord; _e._ parietal mesoblast; _pl._ outline of area pellucida;
_pv._ primitive streak.]

[FIG. 110. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD
OF AN EMBRYO CHICK OF THIRTY HOURS.

_hb._ hind-brain; _vg._ vagus nerve; _ep._ epiblast; _ch._
notochord; _x._ thickening of hypoblast (possibly a rudiment of the
subnotochordal rod); _al._ throat; _ht._ heart; _pp._ body cavity;
_so._ somatic mesoblast; _sf._ splanchnic mesoblast; _hy._
hypoblast.]

[FIG. 111. CHICK OF THE THIRD DAY (54 HOURS) VIEWED FROM UNDERNEATH
AS A TRANSPARENT OBJECT.

_a´._ the outer amniotic fold or false amnion. This is very
conspicuous around the head, but may also be seen at the tail.

_a._ the true amnion, very closely enveloping the head, and here
seen only between the projections of the several cerebral vesicles.
It may also be traced at the tail, _t_.

In the embryo of which this is a drawing the head-fold of the amnion
reached a little farther backward than the reference _u_, but its
limit cannot be distinctly seen through the body of the embryo.

_C.H._ cerebral hemisphere; _F.B._ vesicle of the third ventricle;
_M.B._ mid-brain; _H.B._ hind-brain; _Op._ eye; _Ot._ auditory
vesicle.

_OfV._ vitelline veins forming the venous roots of the heart. The
trunk on the right hand (left trunk when the embryo is viewed in its
natural position from above) receives a large branch, shewn by
dotted lines, coming from the anterior portion of the sinus
terminalis. _Ht._ the heart, now completely twisted on itself. _Ao._
the bulbus arteriosus, the three aortic arches being dimly seen
stretching from it across the throat, and uniting into the aorta,
still more dimly seen as a curved dark line running along the body.
The other curved dark line by its side, ending near the reference
_y_, is the notochord _ch_.

About opposite the line of reference _x_ the aorta divides into two
trunks, which running in the line of the somewhat opaque somites on
either side, are not clearly seen. Their branches however, _Of.a_,
the vitelline arteries, are conspicuous and are seen to curve round
the commencing side-folds.

_Pv._ mesoblastic somites.

_x_ is placed at the "point of divergence" of the splanchnopleure
folds. The blind foregut begins here and extends about up to near
_y_, the more transparent space marked by that letter is however
mainly due to the presence there of investing mass at the base of
the brain. _x_ marks the hind limit of the splanchnopleure folds.
The limit of the more transparent somatopleure folds cannot be seen.

It will be of course understood that all the body of the embryo
above the level of the reference _x_, is seen through the portion of
the yolk-sack (vascular and pellucid area), which has been removed
with the embryo from the egg, as well as through the double amniotic
fold.

The view being from below, whatever is described in the natural
position as being to the right appears here to the left, and _vice
versâ_.]

During the latter half of the second day, and during the third day, great progress is made in the folding off of the embryo. Both the head- and tail-ends of the embryo become quite distinct, and the side-folds make such considerable progress that the embryo is only connected with the yolk by a broad stalk. This stalk is double, and consists of an inner splanchnic stalk, continuous with the walls of the alimentary canal, and an outer somatic stalk, continuous with the body-walls of the embryo. The somatic stalk is very much wider than the splanchnic. (Compare fig. 121 E and F, which may be taken as diagrammatic longitudinal and transverse sections of the embryo on the third day.) A change also takes place in the position of the embryo. Up to the third day it is placed symmetrically, on the yolk, with its ventral face downwards. During this day it turns so as partially to lie on its left side. This rotation affects first the head (fig. 111), but in the course of the fourth day gradually extends to the rest of the body (fig. 118). Coincidently with this change in position the whole embryo undergoes a ventral and somewhat spiral flexure.

During the latter part of the second day and during the third day important changes take place in the head. One of these is the cranial flexure. This, which must not be confounded with the curvature of the body just referred to, commences by the bending downwards of the front part of the head round a point which may be considered as the extreme end either of the notochord or of the alimentary canal.

The cranial flexure progresses rapidly, the front-brain being more and more folded down till, at the end of the third day, it is no longer the first vesicle or fore-brain, but the second cerebral vesicle or mid-brain, which occupies the extreme front of the long axis of the embryo. In fact a straight line through the long axis of the embryo would now pass through the mid-brain instead of, as at the beginning of the second day, through the fore-brain, so completely has the front end of the neural canal been folded over the end of the notochord. The commencement of this cranial flexure gives the body of an embryo of the third day somewhat the appearance of a chemist's retort, the head of the embryo corresponding to the bulb. On the fourth day the flexure is still greater than on the third, but on the fifth and succeeding days it becomes less obvious.

The anterior part of the fore-brain has now become greatly dilated, and may be distinguished from the posterior part as the unpaired rudiment of the cerebral hemispheres. It soon bulges out laterally into two lobes, which do not however become separated by a median partition till a much later period.

[FIG. 112. SIDE VIEW OF THE HEAD OF AN EMBRYO CHICK OF THE THIRD DAY
AS AN OPAQUE OBJECT. (Chromic acid preparation.)

_CH._ Cerebral hemispheres; _F.B._ Vesicle of third ventricle;
_M.B._ Mid-brain; _Cb._ Cerebellum; _H.B._ Medulla oblongata; _N._
Nasal pit; _ot._ auditory vesicle in the stage of a pit with the
opening not yet closed up; _op._ Optic vesicle, with _l._ lens and
_ch.f._ choroidal fissure. The choroidal fissure, though formed
entirely underneath the superficial epiblast, is distinctly visible
from the outside.

_1 F._ The first visceral fold; above it is seen a slight indication
of the superior maxillary process.

_2_, _3_, _4 F._ Second, third and fourth visceral folds, with the
visceral clefts between them.]

Owing to the development of the cerebral rudiment the posterior part of the fore-brain no longer occupies the front position (fig. 111, and 112 _FB_), and ceases to be the conspicuous object that it was. Inasmuch as its walls will hereafter be developed into the parts surrounding the so-called third ventricle of the brain, it is known as the vesicle of the third ventricle, or the thalamencephalon.

On the summit of the thalamencephalon there may now be seen a small conical projection, the rudiment of the _pineal gland_, while the centre of the floor is produced into a funnel-shaped process, the infundibulum, which, stretching towards the extreme end of the alimentary canal, joins the pituitary body.

Beyond an increase in size, which it shares with nearly all parts of the embryo, and the change of position which has already been referred to, the mid-brain undergoes no great alterations during the third day. Its sides will ultimately become developed into the corpora bigemina or optic lobes, its floor will form the crura cerebri, and its cavity will be reduced to the narrow canal known as the iter a tertio ad quartum ventriculum and two diverticula leading from this into the optic lobes.

In the hind-brain, or third cerebral vesicle, the roof of the part which lies nearest to the mid-brain, becomes during the third day marked off from the rest by a slight constriction. This distinction, which becomes much more evident later on by a thickening of the walls and roof of the front portion, separates the hind-brain into the cerebellum and the medulla oblongata (fig. 112 _Cb_ and _HB_). While the walls of the cerebellar portion of the hind-brain become very much thickened as well at the roof as at the sides, the roof of the posterior portion or medulla oblongata thins out into a mere membrane, forming a delicate covering to the cavity of the vesicle (fig. 114 _IV_), which here becoming broad and shallow with greatly thickened floor and sides, is known as the fourth ventricle, subsequently overhung by the largely-developed posterior portion of the cerebellum.

[FIG. 113. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY VIEWED AS AN
OPAQUE OBJECT: FROM THE FRONT IN A, AND FROM THE SIDE IN B. (Chromic
acid preparation.)

_CH._ cerebral hemispheres; _FB._ vesicle of the third ventricle;
_Op._ eyeball; _nf._ nasofrontal process; _M._ cavity of mouth;
_SM._ superior maxillary process of _F. 1_, the first visceral fold
(inferior maxillary process); _F. 2_, _F. 3_, second and third
visceral folds; _N._ nasal pit; _ot._ otic vesicle.

In order to gain the view here given the neck was cut across between
the third and fourth visceral folds. In the section _e_ thus made,
are seen the alimentary canal _al_, the neural canal _n.c._, the
notochord _ch_, the dorsal aorta _AO_, and the vertebral veins _V_.]

The third day, therefore, marks the distinct differentiation of the brain into five distinct parts: the cerebral hemispheres, the central masses round the third ventricle, the corpora bigemina, the cerebellum and the medulla oblongata; the original cavity of the neural canal at the same time passing from its temporary division of three single cavities into the permanent arrangement of a series of connected ventricles, viz. the lateral ventricles, the third ventricle, the iter (with a prolongation into the optic lobe on each side), and the fourth ventricle.

By the third day the lens of the eye has become formed by an invagination of the epiblast, and other changes in the eye have taken place. The external opening of the auditory pit is closed before the completion of the third day (fig. 114, _RL_); and the rudiments of the external parts of the organ of smell have become formed as small pits on the under surface of the fore-brain (fig. 112, _N_). Like the lens and the labyrinth of the ear, they are formed as invaginations of the external epiblast; unlike them they are never closed up.

[FIG. 114. 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.]

During the second and third days there are formed the visceral or branchial clefts, homologous with those of the Ichthyopsida, though never developing branchial processes from their walls.

They are however real clefts or slits passing right through the walls of the throat, and are placed in series on either side across the axis of the alimentary canal, lying not quite at right angles to that axis nor parallel to each other, but converging somewhat to the middle of the throat in front (fig. 112 and fig. 113).

Four in number on either side, the anterior is the first to be formed, the other three following in succession. They originate as pouches of the hypoblast, which meet the epiblast. At the junction of the epiblast and hypoblast an absorption of the tissue is effected, placing the pouches in communication with the exterior.

No sooner has a cleft been formed than its anterior border (_i.e._ the border nearer the head) becomes raised into a thick lip or fold, the _visceral_ or _branchial fold_. Each cleft has its own fold on its anterior border, and in addition the posterior border of the fourth or last visceral cleft is raised into a similar fold. There are thus _five_ visceral folds to _four_ visceral clefts (figs. 112 and 113). The last two folds however, and especially the last, are not nearly so thick and prominent as the other three, the second being the broadest and most conspicuous of all. The first fold meets, or nearly meets, its fellow in the middle line in front, but the second falls short of reaching the middle line, and the third, fourth and fifth do so in an increasing degree. Thus in front views of the neck a triangular space with its apex directed towards the head is observed between the ends of the several folds (fig. 113 A).

Into this space the pleuroperitoneal cavity extends, the somatopleure separating from the splanchnopleure along the ends of the folds; and it is here that the aorta plunges into the mesoblast of the body.

The history of these most important visceral folds and clefts will be dealt with in detail hereafter; meanwhile I may say that in the Chick and higher Vertebrates the first three pairs of folds are those which call for most notice.

The first fold on either side, increasing rapidly in size and prominence, does not, like the others, remain single, but sends off in the course of the third day a branch or bud-like process from its upper edge (fig. 113). This branch, starting from near the outer end of the fold, runs forwards and upwards in front of the stomodæum, tending to meet the corresponding branch from the fold on the other side, at a point in the middle line nearer the front of the head than the junction of the main folds (fig. 113, _sm_). The two branches do not quite meet, being separated by a median process, which at the same time grows down from the extreme front of the head, and against which they abut (fig. 120, _k_). Between the main folds, which are directed somewhat downwards and their branches which slant upwards, the somewhat lozenge-shaped stomodæum is placed, which, as the folds become more and more prominent, grows deeper and deeper (fig. 120 A). The main folds form the mandibular arch, and their branches the maxillary processes, and the descending process which helps to complete the anterior margin of the stomodæum or oral cavity is called, from the parts which will be formed out of it, the _frontonasal process_.

[FIG. 115. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO
CHICK OF 45 HOURS.

_M.c._ medullary canal; _P.v._ mesoblastic somite; _W.d._ Wolffian
duct; _So._ Somatopleure; _S.p._ Splanchnopleure; _p.p._
pleuroperitoneal cavity; _ao._ aorta; _v._ blood-vessels; _w._
germinal wall; _ch._ notochord; _op._ junction between area opaca
and area pellucida.]

In two succeeding pairs of visceral folds, which correspond with the hyoid and first branchial arches of the Ichthyopsida, are developed the parts of the hyoid bone, which will be best considered in connection with the development of the skull. The last two disappear in the Chick without giving rise to any permanent structures. The external opening of the first visceral _i.e._ hyomandibular cleft becomes closed[66], but the inner part of the cleft, opening into the mouth, gives rise to the Eustachian tube and the tympanic cavity, the latter being formed as a special diverticulum.

[66] _Vide_ Moldenhauer, "Die Entwicklung des mittleren und des
äusseren Ohres." _Morphologisches Jahrbuch_, Vol. III. 1877.

Part of the membranous mandibular and hyoid arches form a wall round the dorsal part of the original opening of this cleft, and so give rise to the meatus auditorius externus. At the bottom of this is placed the tympanic membrane, which is probably derived from the tissue which grows over the dorsal part of the opening of the first cleft. It is formed of an external epiblast epithelium, a middle layer of mesoblast, and an internal hypoblastic epithelium.

[FIG. 116. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO
WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.

_am._ amnion; _so._ somatopleure; _sp._ splanchnopleure; _wd._
Wolffian duct; _st._ segmental tube; _ca.v._ cardinal vein; _ms._
muscle-plate; _sp.g._ spinal ganglion; _sp.c._ spinal cord; _ch._
notochord; _ao._ aorta; _hy._ hypoblast.]

The general nature of the changes, which take place in the trunk between the commencement of the second half of the second day and the end of the third day, is illustrated by the sections figs. 115, 116, 117.

[FIG. 117. SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK AT
THE END OF THE THIRD DAY.

_Am._ amnion; _m.p._ muscle-plate. _C.V._ cardinal vein. _Ao._
dorsal aorta. The section passes through the point where the dorsal
aorta is just commencing to divide into two branches. _Ch._
notochord; _W.d._ Wolffian duct; _W.b._ commencing differentiation
of the mesoblast cells to form the Wolffian body; _ep._ epiblast;
_So._ somatopleure; _Sp._ splanchnopleure; _hy._ hypoblast. The
section passes through the point where the digestive canal
communicates with the yolk-sack, and is consequently still _open_
below.]

In the earliest of these sections there is not a trace of a folding off of the embryo from the yolk, and the body walls are quite horizontal. In the second section (fig. 116), from an embryo of about two days, the body walls are already partially inclined, and the splanchnopleure is very distinctly folded inwards. There is a considerable space between the notochord and the hypoblast, which forms the rudiment of the mesentery.

In the third section (fig. 117) the body walls have become nearly vertical, the folding of the splanchnopleure is nearly completed, and it is only for a small region that the alimentary tract is open, by the vitelline duct, to the yolk-sack.

[FIG. 118. EMBRYO CHICK AT THE END OF THE FOURTH DAY SEEN AS A
TRANSPARENT OBJECT.

The amnion has been completely removed, the cut end of the somatic
stalk is shewn at _S.S._ with the allantois (_Al_) protruding from
it.

_C.H._ cerebral hemisphere; _F.B._ vesicle of the third ventricle
with the pineal gland (_Pn_) projecting from its summit; _M.B._
mid-brain; _Cb._ cerebellum. _IV. V._ fourth ventricle; _L._ lens;
_ch.s._ choroid slit. Owing to the growth of the optic cup the two
layers of which it is composed cannot any longer be seen from the
surface, but the retinal surface of the layer alone is visible.
_Cen. V._ auditory vesicle; _s.m._ superior maxillary process; _1
F_, _2 F_, etc. first, second, third and fourth visceral arches;
_V._ fifth nerve sending one branch to the eye, the ophthalmic
branch, and another to the first visceral arch; _VII._ seventh nerve
passing to the second visceral arch; _G.Ph._ glossopharyngeal nerve
passing towards the third visceral arch; _Pg._ pneumogastric nerve
passing towards the fourth visceral arch; _iv._ investing mass. No
attempt has been made in the figure to indicate the position of the
dorsal wall of the throat, which cannot be easily made out in the
living embryo; _ch._ notochord. The front end of this cannot be seen
in the living embryo. It does not end however as shewn in the
figure, but takes a sudden bend downwards and then terminates in a
point. _Ht._ heart seen through the walls of the chest; _M.P._
muscle-plates. _W._ wing; _H.L._ hind limb. Beneath the hind limb is
seen the curved tail.]

These three sections further illustrate (1) the gradual differentiation of the mesoblastic somites (fig. 115, _P.v_) into (_a_) the muscle-plates (figs. 116, _ms_ and 117, _m.p_), and (_b_) the tissue to form the vertebral bodies and adjacent connective tissue; (2) the formation of a mass of tissue between the lateral plates and the mesoblastic somites (fig. 115), known as the intermediate cell mass, on the dorsal side of which the Wolffian duct is formed, while the intermediate cell mass itself breaks up into the segmental tubes (fig. 116, _st_) and connective tissue of the Wolffian body.

[FIG. 119. SECTION THROUGH THE LUMBAR REGION OF AN EMBRYO CHICK AT
THE END OF THE FOURTH DAY.

_n.c._ neural canal; _p.r._ posterior root of spinal nerve with
ganglion; _a.r._ anterior root of spinal nerve; _A.G.C._ anterior
grey column of spinal cord; _A.W.C._ anterior white column of spinal
cord just commencing to be formed, and not very distinctly marked in
the figure; _m.p._ muscle-plate; _ch._ notochord; _W.R._ Wolffian
ridge; _AO._ dorsal aorta; _V.c.a._ posterior cardinal vein; _W.d._
Wolffian duct; _W.b._ Wolffian body, consisting of tubules and
Malpighian bodies; _g.e._ germinal epithelium; _d._ alimentary
canal; _M._ commencing mesentery; _SO._ somatopleure; _SP._
splanchnopleure; _V._ blood-vessels; _pp._ pleuroperitoneal cavity.]

Various other features in the development of the vascular system, general mesoblast, etc., are also represented in these sections. It may more especially be noted that there are at first two widely separated dorsal aortæ, which gradually approach (figs. 115 and 116); and meeting first of all in front finally coalesce (figs. 117 and 119) for their whole length.

The general appearance of the embryo of the fourth day may be gathered from fig. 118.

[FIG. 120. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS
OF INCUBATION. (From Huxley.)

_I^a._ cerebral vesicles; _a._ eye, in which the remains of the
choroid slit can still be seen in A; _g._ nasal pits; _k._
frontonasal process; _l._ superior maxillary process; _1._ inferior
maxillary process or first visceral arch; _2._ second visceral arch;
_x._ first visceral cleft.

In A the cavity of the mouth is seen enclosed by the frontonasal
process, the superior maxillary processes and the first pair of
visceral arches. At the back of it is seen the opening leading into
the throat. The nasal grooves leading from the nasal pits to the
mouth are already closed over and converted into canals.

In B the external opening of the mouth has become much constricted,
but it is still enclosed by the frontonasal process and superior
maxillary processes above, and by the inferior maxillary processes
(first pair of visceral arches) below.

The superior maxillary processes have united with the frontonasal
process, along nearly the whole length of the latter.]

The changes which have taken place consist for the most part in the further development of the parts already present, and do not need to be specified in detail. The most important event of the day is perhaps the formation of the limbs. They appear as outgrowths from a slightly marked lateral ridge (fig. 119, _WR_), which runs on the level of the lower end of the muscle-plates for nearly the whole length of the trunk. This ridge is known as the Wolffian ridge. The first trace of the limbs can be seen towards the end of the third day; and their appearance at the end of the fourth day is shown in fig. 118, _W_ and _HL_.

A section through the trunk of the embryo on the fourth day is represented in fig. 119. The section passes through the region of the trunk behind the vitelline duct. The mesentery (_M_) is very much deeper and thinner than on the previous day. The notochord has become invested by a condensed mesoblastic tissue, which will give rise to the vertebral column. The two dorsal aortæ have now completely coalesced into the single dorsal aorta, and the Wolffian body has reached a far more complete development.

In the course of the fifth day the face begins to assume a less embryonic character, and by the sixth and succeeding days presents distinctive avian characters.

The general changes which take place between the sixth day and the time of hatching do not require to be specified in detail.

_Foetal Membranes._

The Reptilia, Aves and Mammalia are distinguished from the Ichthyopsida by the possession of certain provisional foetal membranes, known as the amnion and allantois.

As the mode of development of these membranes may be most conveniently studied in the Chick, I have selected this type for their detailed description.

The Amnion. The amnion is a peculiar sack which envelopes and protects the embryo.

At the end of the first day of incubation, when the cleavage of the mesoblast has somewhat advanced, there appears, a little way in front of the semilunar head-fold, a second fold (fig. 102, also fig. 121 C, _af_ and fig. 122, _Am_), running more or less parallel or rather concentric with the first and not unlike it in general appearance, though differing widely from it in nature. This second fold gives rise to the amnion, and is limited entirely to the somatopleure. Rising up as a semilunar fold with its concavity directed towards the embryo (fig. 121 C, _af_), as it increases in height it is gradually drawn backwards over the developing head of the embryo. The fold thus covering the head is in due time accompanied by similar folds of somatopleure, starting at some little distance behind the tail, and at some little distance from the side (fig. 121 C, D, E, F, and 116, _am_). In this way the embryo becomes surrounded by a series of folds of thin somatopleure, which form a continuous wall all round it. All are drawn gradually over the body of the embryo, and at last meet and completely coalesce (fig. 121, H, I, and 117, _Am_), all traces of their junction being removed. Beneath these united folds there is therefore a cavity, within which the embryo lies (fig. 121 H, _ae_). This cavity is the cavity of the amnion.

[FIG. 121. A to N forms a series of purely diagrammatic
representations introduced to facilitate the comprehension of the
manner in which the body of the embryo is formed, and of the various
relations of the yolk-sack, amnion, and allantois.

In all _vt_ is the vitelline membrane, placed, for convenience sake,
at some distance from its contents, and represented as persisting in
the later stages; in reality it is in direct contact with the
blastoderm or yolk, and early ceases to have a separate existence.
In all _e_ indicates the embryo proper; _pp_ the general
pleuroperitoneal space with its extension between the membranes;
_af_ the folds of the amnion; _a_ the amnion proper; _ae_ or _ac_
the cavity holding the liquor amnii; _al_ the allantois; _a´_ the
alimentary canal; _y_ or _ys_ the yolk or yolk-sack.

A, which may be considered as a vertical section taken
longitudinally along the axis of the embryo, represents the
relations of the parts of the egg at the time of the first
appearance of the head-fold, seen on the right-hand side of the
embryo _e_. The blastoderm is spreading both behind (to the left
hand in the figure), and in front (to right hand) of the head-fold,
its limits being indicated by the shading and thickening for a
certain distance of the margin of the yolk _y_. As yet there is no
fold on the left side of _e_ corresponding to the head-fold on the
right.

B is a vertical transverse section of the same period drawn for
convenience sake on a larger scale (it should have been made flatter
and less curved). It shews that the blastoderm (vertically shaded)
is extending laterally as well as fore and aft, in fact in all
directions; but there are no lateral folds, and therefore no lateral
limits to the body of the embryo as distinguished from the
blastoderm.

Incidentally it shews the formation of the medullary groove by the
rising up of the laminæ dorsales. Beneath the section of the groove
is seen the rudiment of the notochord. On either side a line
indicates the cleavage of the mesoblast just commencing.

In C, which represents a vertical longitudinal section of later
date, both head-fold (on the right) and tail-fold (on the left) have
advanced considerably. The alimentary canal is therefore closed in,
both in front and behind, but is in the middle still widely open to
the yolk _y_ below. Though the axial parts of the embryo have become
thickened by growth, the body-walls are still thin; in them however
is seen the cleavage of the mesoblast, and the divergence of the
somatopleure and splanchnopleure. The splanchnopleure both at the
head and at the tail is folded in to a greater extent than the
somatopleure, and forms the still wide splanchnic stalk. At the end
of the stalk, which is as yet short, it bends outwards again and
spreads over the surface of the yolk. The somatopleure, folded in
less than the splanchnopleure to form the wider somatic stalk,
sooner bends round and runs outwards again. At a little distance
from both the head and the tail it is raised up into a fold, _af_,
_af_, that in front of the head being the highest. These are the
amniotic folds. Descending from either fold, it speedily joins the
splanchnopleure again, and the two, once more united into an uncleft
membrane, extend some way downwards over the yolk, the limit or
outer margin of the opaque area not being shewn. All the space
between the somatopleure and the splanchnopleure is shaded with
dots, _pp_. Close to the body this space may be called the
pleuroperitoneal cavity; but outside the body it runs up into either
amniotic fold, and also extends some little way over the yolk.

D represents the tail end at about the same stage on a more enlarged
scale, in order to illustrate the position of the allantois _al_
(which was for the sake of simplicity omitted in C), shewn as a bud
from the splanchnopleure, stretching downwards into the
pleuroperitoneal cavity _pp_. The dotted area representing as before
the whole space between the splanchnopleure and the somatopleure, it
is evident that a way is open for the allantois to extend from its
present position into the space between the two limbs of the
amniotic fold _af_.

E, also a longitudinal section, represents a stage still farther
advanced. Both splanchnic and somatic stalks are much narrowed,
especially the former, the cavity of the alimentary canal being now
connected with the cavity of the yolk by a mere canal. The folds of
the amnion are spreading over the top of the embryo and nearly meet.
Each fold consists of two walls or limbs, the space between which
(dotted) is as before merely a part of the space between the
somatopleure and splanchnopleure. Between these arched amniotic
folds and the body of the embryo is a space not as yet entirely
closed in.

F represents on a different scale a transverse section of E taken
through the middle of the splanchnic stalk. The dark ring in the
body of the embryo shews the position of the neural canal, below
which is a black spot, marking the notochord. On either side of the
notochord the divergence of somatopleure and splanchnopleure is
obvious. The splanchnopleure, more or less thickened, is somewhat
bent in towards the middle line, but the two sides do not unite, the
alimentary canal being as yet open below at this spot; after
converging somewhat they diverge again and run outwards over the
yolk. The somatopleure, folded in to some extent to form the
body-walls, soon bends outwards again, and is almost immediately
raised up into the lateral folds of the amnion _af_. The continuity
of the pleuroperitoneal cavity, within the body, with the interior
of the amniotic fold, outside the body, is evident; both cavities
are dotted.

G, which corresponds to D at a later stage, is introduced to shew
the manner in which the allantois, now a considerable hollow body,
whose cavity is continuous with that of the alimentary canal,
becomes directed towards the amniotic fold.

In H a longitudinal, and I a transverse section of later date, great
changes have taken place. The several folds of the amnion have met
and coalesced above the body of the embryo. The inner limbs of the
several folds have united into a single membrane (_a_), which
encloses a space (_ae_ or _ac_) round the embryo. This membrane _a_
is the amnion proper, and the cavity within it, _i.e._ between it
and the embryo, is the cavity of the amnion containing the liquor
amnii. The allantois is omitted for the sake of simplicity.

It will be seen that the amnion _a_ now forms in every direction the
termination of the somatopleure; the peripheral portions of the
somatopleure, the united outer or descending limbs of the folds _af_
in C, D, F, G having been cut adrift, and now forming an independent
continuous membrane, the serous membrane, immediately underneath the
vitelline membrane.

In I the splanchnopleure is seen converging to complete the closure
of the alimentary canal _a´_ even at the stalk (elsewhere the canal
has of course long been closed in), and then spreading outwards as
before over the yolk. The point at which it unites with the
somatopleure, marking the extreme limit of the cleavage of the
mesoblast, is now much nearer the lower pole of the diminished yolk.

As a result of these several changes, a great increase in the dotted
space has taken place. It is now possible to pass from the actual
peritoneal cavity within the body, on the one hand round a great
portion of the circumference of the yolk, and on the other hand
above the amnion _a_, in the space between it and the serous
envelope.

Into this space the allantois is seen spreading in K at _al_.

In L the splanchnopleure has completely invested the yolk-sack, but
at the lower pole of the yolk is still continuous with that
peripheral remnant of the somatopleure now called the serous
membrane. In other words, cleavage of the mesoblast has been carried
all round the yolk (_ys_) except at the very lower pole.

In M the cleavage has been carried through the pole itself; the
peripheral portion of the splanchnopleure forms a complete
investment of the yolk quite unconnected with the peripheral portion
of the somatopleure, which now exists as a continuous membrane
lining the interior of the shell. The yolk-sack (_ys_) is therefore
quite loose in the pleuroperitoneal cavity, being connected only
with the alimentary canal (_a´_) by a solid pedicle.

Lastly, in N the yolk-sack (_ys_) is shewn being withdrawn into the
cavity of the body of the embryo. The allantois is as before, for
the sake of simplicity, omitted; its pedicle would of course lie by
the side of _ys_ in the somatic stalk marked by the usual dotted
shading.

It may be repeated that the above are diagrams, the various spaces
being shewn distended, whereas in many of them in the actual egg the
walls have collapsed, and are in near juxtaposition.]

[FIG. 122. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN
EMBRYO.

The section is supposed to be made at a time when the head-fold has
commenced but the tail-fold has not yet appeared.

_F.So._ fold of the somatopleure. _F.Sp._ fold of the
splanchnopleure; _D._ foregut.

_pp._ pleuroperitoneal cavity between somatopleure and
splanchnopleure; _Am._ commencing (head) fold of the amnion. For
remaining reference letters _vide_ p. 167.]

Each fold is necessarily formed of two limbs, both limbs consisting of epiblast and a very thin layer of mesoblast; but in one limb the epiblast looks towards the embryo, while in the other it looks away from it. The space between the two limbs of the fold, as can easily be seen in fig. 121, is really part of the space between the somatopleure and splanchnopleure; it is therefore continuous with the general space, part of which afterwards becomes the pleuroperitoneal cavity of the body, shaded with dots in the figure and marked (_pp_); so that it is possible to pass from the cavity between the two limbs of the amniotic folds into the cavity which surrounds the alimentary canal. When the several folds meet and coalesce together above the embryo, they unite in such a way that all their inner limbs unite to form a continuous inner membrane or sack, and all their outer limbs a similarly continuous outer membrane or sack. The inner membrane thus built up forms a completely closed sack round the body of the embryo, and is called the amniotic sack, or _amnion proper_ (fig. 121, H, I, &c., _a_), and the fluid which it afterwards contains is called the amniotic fluid, or _liquor amnii_. The space between the inner and outer sack is, from the mode of its formation, simply a part of the general cavity found everywhere between somatopleure and splanchnopleure. The outer sack over the embryo lies close under the vitelline membrane, and the cavity between it and the true amnion is gradually extended over the whole yolk-sack.

The actual manner in which the amniotic folds meet is somewhat peculiar (His and Kölliker). The head-fold of the amnion is the earliest formed, and completely covers over the head before the end of the second day. The side and tail folds are later in developing. The side-folds finally meet in the dorsal line, and their coalescence proceeds backwards from the head-fold in a linear direction, till there is only a small opening left over the tail. This also becomes closed early on the third day.

The allantois[67] is essentially a diverticulum of the alimentary tract into which it opens immediately in front of the anus. Its walls are formed of splanchnic mesoblast with blood-vessels, within which is a lining of hypoblast. It becomes a conspicuous object on the third day of incubation, but its first development takes place at an earlier period, and is intimately connected with the formation of the posterior section of the gut.

[67] For details on the development of the allantois the reader
is referred to the works of Kölliker (No. 135), Gasser (No. 127),
and for a peculiar view on the subject Kupffer (No. 136). In
addition to these works he may refer to Dobrynin "Ueber die erste
Anlage der Allantois." _Sitz. der k. Akad. Wien_, Bd. 64, 1871.
E. Gasser, _Beiträge zur Entwicklungsgeschichte d. Allantois_,
etc.

At the time of the folding in of the hinder end of the mesenteron the splitting of the mesoblast into somatopleure and splanchnopleure has extended up to the border of the hinder division of the primitive streak. As has been already mentioned, the ventral wall of the postanal section of the alimentary tract is formed by the primitive streak. Immediately in front of this is the involution which forms the proctodæum; while the wall of the hindgut in front of the anus owes its origin to a folding in of the splanchnopleure.

The allantois first appears as a protuberance of the splanchnopleure just in front of the anus. This protuberance arises, however, before the splanchnopleure has begun to be tucked in so as to form the ventral wall of the hindgut; and it then forms a diverticulum (fig. 123 A, _All_) the open end of which is directed forward, while its blind end points somewhat upwards and towards the peritoneal space behind the embryo.

[FIG. 123. TWO LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EMBRYO
CHICK TO SHEW THE ORIGIN OF THE ALLANTOIS. A AT THE BEGINNING OF THE
THIRD DAY; B AT THE MIDDLE OF THE THIRD DAY. (After Dobrynin.)

_t._ the tail; _m._ the mesoblast of the body, about to form the
mesoblastic somites; _x´._ the roof of _x´´._ the neural canal;
_Dd._ the hind end of the hindgut; _So._ somatopleure; _Spl._
splanchnopleure; _u._ the mesoblast of the splanchnopleure carrying
the vessels of the yolk-sack; _pp._ pleuroperitoneal cavity; _Df._
the epithelium lining the pleuroperitoneal cavity; _All._ the
commencing allantois; _w._ projection formed by anterior and
posterior divisions of the primitive streak; _y._ hypoblast which
will form the ventral wall of the hindgut; _v._ anal invagination;
_G._ cloaca.]

As the hindgut becomes folded in the allantois shifts its position, and forms (figs. 123 B and 124) a rather wide vesicle lying immediately below the hind end of the digestive canal, with which it communicates freely by a still considerable opening; its blind end projects into the pleuroperitoneal cavity below.

Still later the allantois grows forward, and becomes a large spherical vesicle, still however remaining connected with the cloaca by a narrow canal which forms its neck or stalk (fig. 121 G, _al_). From the first the allantois lies in the pleuroperitoneal cavity. In this cavity it grows forwards till it reaches the front limit of the hindgut, where the splanchnopleure turns back to enclose the yolk-sack. It does not during the third day project beyond this point; but on the fourth day begins to pass out beyond the body of the chick, along the as yet wide space between the splanchnic and somatic stalks of the embryo, on its way to the space between the external and internal folds of the amnion, which it will be remembered is directly continuous with the pleuroperitoneal cavity (fig. 121 K). In this space it eventually spreads out over the whole body of the chick. On the first half of the fourth day the vesicle is still very small, and its growth is not very rapid. Its mesoblast wall still remains very thick. In the latter half of the day its growth becomes very rapid, and it forms a very conspicuous object in a chick of that date (fig. 118, _Al_). At the same time its blood-vessels become important. It receives its supply of blood from two branches of the iliac arteries known as the allantoic arteries[68], and the blood is brought back from it by two allantoic veins which run along in the body walls (fig. 119) and after uniting into a single trunk fall into the vitelline vein close behind the liver.

[68] I propose to call these arteries and the corresponding veins
the allantoic arteries and veins, instead of using the confusing
term 'umbilical.'

[FIG. 124. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

_ep._ epiblast; _Sp.c._ spinal canal; _ch._ notochord; _n.e._
neurenteric canal; _hy._ hypoblast; _p.a.g._ postanal gut; _pr._
remains of primitive streak folded in on the ventral side; _al._
allantois; _me._ mesoblast; _an._ point where anus will be formed;
_p.c._ perivisceral cavity; _am._ amnion; _so._ somatopleure; _sp._
splanchnopleure.]

Before dealing with the later history of the foetal membranes, it will be convenient to complete the history of the yolk-sack.

Yolk-Sack. The origin of the area opaca has already been described. It rapidly extends over the yolk underneath the vitelline membrane; and is composed of epiblast and of the hypoblast of the germinal wall continuous with that of the area pellucida, which on the fourth day takes the form of a more or less complete layer of columnar cells[69]. Between the epiblast and hypoblast there is a layer of mesoblast, which does not extend as far as the two other layers. The yolk is completely surrounded by the seventh day.

[69] Further investigations are required as to the character of
this layer.

[FIG. 125. DIAGRAM OF THE CIRCULATION OF THE YOLK-SACK AT THE END OF
THE THIRD DAY OF INCUBATION.

_H._ heart; _AA._ the second, third and fourth aortic arches; the
first has become obliterated in its median portion, but is continued
at its proximal end as the external carotid, and at its distal end
as the internal carotid; _AO._ dorsal aorta; _L.Of.A._ left
vitelline artery; _R.Of.A._ right vitelline artery; _S.T._ sinus
terminalis; _L.Of._ left vitelline vein; _R.Of._ right vitelline
vein; _S.V._ sinus venosus; _D.C._ ductus Cuvieri; _S.Ca.V._
superior cardinal vein; _V.Ca._ inferior cardinal vein. The veins
are marked in outline and the arteries are black. The whole
blastoderm has been removed from the egg and is supposed to be
viewed from below. Hence the left is seen on the right, and _vice
versâ_.]

Towards the end of the first day blood-vessels begin to be developed in the inner part of the mesoblast of the area opaca. Their development is completed on the second day; and the region through which they extend is known as the area vasculosa. The area vasculosa also grows round the yolk, and completely encloses it not long after the area opaca. The part of the blastoderm which thus encloses the yolk forms the yolk-sack. The splitting of the mesoblast gradually extends to the mesoblast of the yolk-sack, and eventually the somatopleure of the sack, which is continuous, it will be remembered, with the outer limb of the amnion, separates completely from the splanchnopleure; and between the two the allantois inserts itself. These features are represented in fig. 121 E, K, and L.

The circulation of the yolk-sack is most important during the third day of incubation. The arrangement of the vessels during that day is shewn in fig. 125.

The blood leaving the body of the embryo by the vitelline arteries (fig. 125, _R.Of.A_, _L.Of.A_), which are branches of the dorsal aortæ, is carried to the small vessels and capillaries of the vascular area, a small portion only being appropriated by the pellucid area.

From the vascular area part of the blood returns directly to the sinus venosus by the main lateral trunks of the vitelline veins (_R.Of_, _L.Of_), and so to the heart. During the second day these venous trunks join the body of the embryo considerably in front of, that is nearer, the head than the corresponding arterial ones. Towards the end of the third day, owing to the continued lengthening of the heart, the veins and arteries run not only parallel to each other, but almost in the same line, the points at which they respectively join and leave the body being nearly at the same distance from the head.

The rest of the blood brought by the vitelline arteries finds its way into the lateral portions of a venous trunk bounding the vascular area, which is known as the sinus terminalis, _S.T._, and there divides on each side into two streams. Of these, the two which, one on either side, flow backward, meet at a point about opposite to the tail of the embryo, and are conveyed along a distinct vein which, running straight forward parallel to the axis of the embryo, empties itself into the left vitelline vein. The two forward streams reaching a gap in the front part of the sinus terminalis fall into either one, or in some cases two veins, which run straight backwards parallel to the axis of the embryo, and so reach the roots of the heart. When one such vein only is present it joins the left vitelline trunk; where there are two they join the left and right vitelline trunks respectively. The left vein is always considerably larger than the right; and the latter when present rapidly gets smaller and speedily disappears. After the third day, although the vascular area goes on increasing in size until it finally all but encompasses the yolk, the prominence of the sinus terminalis becomes less and less.

The foetal membranes and the yolk-sack may conveniently be treated of together in the description of their later changes and final fate.

On the sixth and seventh days they exhibit changes of great importance.