Chapter II: Urochorda5 (2)

[12] Brooks takes a very different view of the nature of the
parts in Salpa. He says, No. 7, p. 322, "The atrium of Salpa,
when first observed, was composed of two broad lateral atria
within the body cavity, one on each side of the branchial sack,
and a very small mid-atrium.... The lateral atria do not however,
as in most Tunicata, remain connected with the mid-atrium, and
unite with the wall of the branchial sack to form the branchial
slits, but soon become entirely separated, and the two walls of
each unite so as to form a broad sheet of tissue, which soon
splits up to form the muscular bands of the branchial sack."
Again, p. 324, "During the changes which have been described as
taking place in the lateral atria, the mid-atrium has increased
in size.... The branchial and atrial tunics now unite upon each
side, so that the sinus is converted into a tube which
communicates, at its posterior end, with the heart and
perivisceral sinus, and at the anterior end with the neural
sinus. This tube is the gill.... The centres of the two regions
upon the sides of the gill, where these two tissues have become
united, are now absorbed, so that a single long and narrow
branchial slit is produced on each side of the gill. The
branchial cavity is thus thrown into communication with the
atrium, and the upper surface of the latter now unites with the
outer tunic, and the external atrial opening is formed by
absorption."

The above description would imply that the atrial cavity is a
space lined by mesoblast, a view which would upset the whole
morphology of the Ascidians. Salensky's account, which implies
only an immense reduction in the size of the atrial cavity as
compared with other types, appears to me far more probable. The
lateral atria of Brooks appear to be simply parts of the body
cavity, and have certainly no connection with the lateral atria
of simple Ascidians or Pyrosoma.

The observations of Todaro upon Salpa (No. 38) are very
remarkable, and illustrated by beautifully engraved plates. His
interpretations do not however appear quite satisfactory. The
following is a brief statement of some of his results.

During segmentation there arises a layer of small superficial
cells (epiblast) and a central layer of larger cells, which
becomes separated from the former by a segmentation cavity,
except at the pole adjoining the free end of the brood-pouch. At
this point the epiblast cells become invaginated into the central
cells and form the alimentary tract, while the primitive central
cells remain as the mesoblast. A fold arises from the epiblast
which Todaro compares to the vertebrate amnion, but the origin of
it is unfortunately not satisfactorily described. The folds of
the amnion project towards the placenta, and enclose a cavity
which, as the folds never completely meet, is permanently open to
the maternal blood sinus. This cavity corresponds with the cavity
of the true amnion of higher Vertebrates. It forms the cavity of
the placenta already described. Between the two folds of the
amnion is a cavity corresponding with the vertebrate false
amnion. A structure regarded by Todaro as the notochord is formed
on the neck, connecting the involution of the alimentary tract
with the exterior. It has only a very transitory existence.

In the later stages the segmentation cavity disappears and a true
body cavity is formed by a split in the mesoblast.

Todaro's interpretations, and in part his descriptions also, both
with reference to the notochord and amnion, appear to me quite
inadmissible. About some other parts of his descriptions it is
not possible to form a satisfactory judgment. He has recently
published a short paper on this subject (No. 39) preliminary to a
larger memoir, which is very difficult to understand in the
absence of plates. He finds however in the placenta various parts
which he regards as homologous with the decidua vera and reflexa
of Mammalia.

Development of the chain of sexual Salps. My description of the embryonic development of Salpa would not be complete without some reference to the development of the stolon of the Solitary generation of Salps by the segmentation of which a chain of sexual Salps originates.

The asexual Salp, the embryonic development of which has just been described, may be compared to the Cyathozooid of Pyrosoma, from which it mainly differs in being fully developed. While still in an embryonic condition it gives rise to a process or stolon, which becomes divided into a number of zooids by transverse constrictions, in the same manner that part of the germ of the ovum of Pyrosoma is divided by transverse constrictions into four Ascidiozooids.

The stolon arises as a projection on the right side of the body of the embryo close to the heart. It is formed (Salensky, No. 35) of an outgrowth of the body wall, into which there grow the following structures:

(1) A central hollow process from the end of the respiratory sack.
(2) A right and left lateral prolongation of the pericardial cavity.
(3) A solid process of cells on the ventral side derived from the
same mass of the cells as the elæoblast.
(4) A ventral and a dorsal blood sinus.

Besides these parts there appears on the dorsal side a hollow tube, the origin of which is unknown, which gives rise to the nervous system.

The hollow process of the respiratory sack is purely provisional, and disappears without giving rise to any permanent structure. The right and left prolongations of the pericardial cavity become solid and eventually give origin to the mesoblast. The ventral process of cells is the most important structure in the stolon in that it gives rise both to the alimentary and respiratory sacks, and to the generative organs of the sexual Salps. The stolon containing the organs just enumerated becomes divided by transverse constrictions into a number of rings. These rings do not long remain complete, but become interrupted dorsally and ventrally. The imperfect rings so formed soon overlap, and each of them eventually gives rise to a sexual Salp. Although the stolon arises while the asexual Salp is still in an embryonic condition, it does not become fully developed till long after the asexual Salp has attained maturity.

Appendicularia. Our only knowledge of the development of Appendicularia is derived from Fol's memoir on the group (No. 8). He simply states that it develops, as far as he was able to follow, like other Ascidians; and that the extremely minute size of the egg prevented him from pursuing the subject. He also states that the pair of pores leading from the branchial cavity to the exterior is developed from epiblastic involutions meeting outgrowths of the wall of the branchial sack.

_Metagenesis._

One of the most remarkable phenomena in connection with the life history of many Ascidians is the occurrence of an alternation of sexual and gemmiparous generations. This alternation appears to have originated from a complication of the process of reproduction by budding, which is so common in this group. The mode in which this very probably took place will be best understood by tracing a series of transitional cases between simple budding and complete alternations of generations.

In the simpler cases, which occur in some Composita Sedentaria, the process of budding commences with an outgrowth of the body wall into the common test, containing a prolongation of part of the alimentary tract[13].

[13] It is not within the scope of this work to enter into
details with reference to the process of budding. The reader is
referred on this head more especially to the papers of Huxley
(No. 16) and Kowalevsky (No. 22) on Pyrosoma, of Salensky (No.
35) on Salpa, and Kowalevsky (No. 21) on Ascidians generally. It
is a question of very great interest how budding first arose, and
then became so prevalent in these degenerate types of Chordata.
It is possible to suppose that budding may have commenced by the
division of embryos at an early stage of development, and have
gradually been carried onwards by the help of natural selection
till late in life. There is perhaps little in the form of budding
of the Ascidians to support this view--the early budding of
Didemnum as described by Gegenbaur being the strongest evidence
for it--but it fits in very well with the division of the embryo
in Lumbricus trapezoides described by Kleinenberg, and with the
not unfrequent occurrence of double monsters in Vertebrata which
may be regarded as a phenomenon of a similar nature (Rauber). The
embryonic budding of Pyrosoma, which might perhaps be viewed as
supporting the hypothesis, appears to me not really in favour of
it; since the Cyathozooid of Pyrosoma is without doubt an
extremely modified form of zooid, which has obviously been
specially developed in connection with the peculiar reproduction
of the Pyrosomidæ.

Between the epiblastic and hypoblastic layers of the bud so formed, a mesoblastic and sometimes a generative outgrowth of the parent also appears.

The systems of organs of the bud are developed from the corresponding layers to those in the embryo[14]. The bud eventually becomes detached, and in its turn gives rise to fresh buds. Both the bud and its parent reproduce sexually as well as by budding: the new colonies being derived from sexually produced embryos.

[14] The atrial spaces form somewhat doubtful exceptions to the
rule.

The next stage of complication is that found in Botryllus (Krohn, Nos. 25 and 26). The larva produced sexually gives rise to a bud from the right side of the body close to the heart. On the bud becoming detached the parent dies away without developing sexual organs. The bud of the second generation gives rise to two buds, a right one and a left one, and like the larva dies without reaching sexual maturity. The buds of the third generation each produce two buds and then suffer the same fate as their parent.

The buds of the third generation arrange themselves with their cloacal extremities in contact, and in the fourth generation a common cloaca is formed, and so a true radial system of zooids is established; the zooids of which are not however sexual.

The buds of the fourth generation in their turn produce two or three buds and then die away.

Fresh systems become formed by a continuation of the process of budding, but the zooids of the secondary systems so formed are sexual. The ova come to maturity before the spermatozoa, so that cross fertilization takes place.

In Botryllus we have clearly a rudimentary form of alternations of generations, in that the sexually produced larva is asexual, and, after a series of asexual generations, produced gemmiparously, there appear sexual generations, which however continue to reproduce themselves by budding.

The type of alternations of generations observable in Botryllus becomes, as pointed out by Huxley, still more marked in Pyrosoma.

The true product of the ovum is here (_vide_ p. 25) a rudimentary individual called by Huxley the Cyathozooid. This gives rise, while still an embryo, by a process equivalent to budding to four fully developed zooids (Ascidiozooids) similar to the parent form, and itself dies away. The four Ascidiozooids form a fresh colony, and reproduce (1) sexually, whereby fresh colonies are formed, and (2) by ordinary budding, whereby the size of the colony is increased. All the individuals of the colony are sexual.

The alternation of generations in Pyrosoma widely differs from that in Botryllus in the fact of the Cyathozooid differing so markedly in its anatomical characters from the ordinary zooids.

In Salpa the process is slightly different[15]. The sexual forms are _now incapable of budding_, and, although at first a series of sexual individuals are united together in the form of a chain, so as to form a colony like Pyrosoma or Botryllus, yet they are so loosely connected that they separate in the adult state. As in Botryllus, the ova are ripe before the spermatozoa. Each sexual individual gives rise to a single offspring, which, while still in the embryonic condition, buds out a 'stolon' from its right ventral side. This stolon is divided into a series of lateral buds after the solitary asexual Salp has begun to lead an independent existence. The solitary asexual Salp clearly corresponds with the Cyathozooid of Pyrosoma, though it has not, like the Cyathozooid, undergone a retrogressive metamorphosis.

[15] _Vide_ p. 33.

By far the most complicated form of alternation of generations known amongst the Ascidians is that in Doliolum. The discovery of this metamorphosis was made by Gegenbaur (No. 10). The sexual form of Doliolum is somewhat cask-shaped, with ring-like muscular bands, and the oral and atrial apertures placed at opposite ends of the cask. The number of gill slits varies according to the species. The ovum gives rise, as already described, to a tailed embryo which subsequently develops into a cask-shaped asexual form. On attaining its full size it loses its branchial sack and alimentary tract. While still in the embryonic condition, a stolon grows out from its dorsal side in the seventh intermuscular space. The stolon, like that in Salpa, contains a prolongation of the branchial sack[16].

[16] I draw this conclusion from Gegenbaur's fig. (No. 10), Pl.
XVI., fig. 15. The body (_x_) in the figure appears to me without
doubt the rudiment of the stolon, and not, as believed by
Gegenbaur, the larval tail.

On this stolon there develop two entirely different types of buds, (1) lateral buds, (2) dorsal median buds.

The lateral buds are developed in regular order on the two sides of the stolon, and the most advanced buds are those furthest removed from the base. They give rise to forms with a very different organization to that of the parent. They are compared by Gegenbaur to a spoon, the bowl of which is formed by the branchial sack, and the handle by the stalk attaching the bud to the stolon. The oral opening into the branchial sack is directed upwards: _an atrial opening is remarkably enough not present_. The branchial sack is perforated by numerous openings. It leads into an alimentary tract which opens directly to the exterior by an anus opposite the mouth.

The stalks attaching the more mature buds to the stolon are provided with ventrally directed scales, which completely hide the stolon in a view from the ventral surface.

These buds have, even after their detachment, no trace of generative organs, and shew no signs of reproducing themselves by budding. Their eventual fate is unknown.

The median dorsal buds have no such regular arrangement as the lateral buds, but arise in irregular bunches, those furthest removed from the base of the stolon being however the oldest. These buds are almost exactly similar to the original sexual form; they do not acquire sexual organs, but are provided with a stolon attached on the ventral side, in the sixth intermuscular space.

This stolon is simply the stalk by which each median bud was primitively attached to the stolon of the first asexual form.

From the stolon of the median buds of the second generation buds are developed which grow into the sexual forms.

The generations of Doliolum may be tabulated in the following way.

Sexual generation,
|
1st asexual form with dorsal stolon,
_________________|_____________________
| |
spoon-like forms developed as 2nd asexual forms developed as
lateral buds (eventual history median buds with ventral stolon,
unknown). |
sexual generation.

BIBLIOGRAPHY.

(6) P. J. van Beneden. "Recherches s. l'Embryogénie, l'Anat. et la Physiol. des Ascidies simples." _Mém. Acad. Roy. de Belgique_, Tom. XX.

(7) W. K. Brooks. "On the development of Salpa." _Bull. of the Museum of Comp. Anat. at Harvard College, Cambridge, Mass._

(8) H. Fol. _Etudes sur les Appendiculaires du détroit de Messine_. Genève et Bâle, 1872.

(9) Ganin. "Neue Thatsachen a. d. Entwicklungsgeschichte d. Ascidien." _Zeit. f. wiss. Zool._, Vol. XX. 1870.

(10) C. Gegenbaur. "Ueber den Entwicklungscyclus von Doliolum nebst Bemerkungen über die Larven dieser Thiere." _Zeit. f. wiss. Zool._, Bd. VII. 1856.

(11) A. Giard. "Etudes critiques des travaux d'embryogénie relatifs à la parenté des Vertebrés et des Tuniciers." _Archiv Zool. expériment._, Vol. I. 1872.

(12) A. Giard. "Recherches sur les Synascidies." _Archiv Zool. expér._, Vol. I. 1872.

(13) O. Hertwig. "Untersuchungen üb. d. Bau u. d. Entwicklung des Cellulose-Mantels d. Tunicaten." _Jenaische Zeitschrift_, Bd. VII. 1873.

(14) Th. H. Huxley. "Remarks upon Appendicularia and Doliolum." _Phil. Trans._, 1851.

(15) Th. H. Huxley. "Observations on the anatomy and physiology of Salpa and Pyrosoma." _Phil. Trans._, 1851.

(16) Th. H. Huxley. "Anatomy and development of Pyrosoma." _Linnean Trans._, 1860, Vol. XXIII.

(17) Keferstein u. Ehlers. _Zoologische Beiträge_, 1861. Doliolum.

(18) A. Kowalevsky. "Entwicklungsgeschichte d. einfachen Ascidien." _Mém. Acad. Pétersbourg_, VII. série, T. X. 1866.

(19) A. Kowalevsky. "Beitrag z. Entwick. d. Tunicaten." _Nachrichten d. königl. Gesell. zu Göttingen._ 1868.

(20) A. Kowalevsky. "Weitere Studien üb. d. Entwicklung d. einfachen Ascidien." _Archiv f. mikr. Anat._, Vol. VII. 1871.

(21) A. Kowalevsky. "Ueber Knospung d. Ascidien." _Archiv f. mikr. Anat._, Vol. X. 1874.

(22) A. Kowalevsky. "Ueber die Entwicklungsgeschichte d. Pyrosoma." _Archiv f. mikr. Anat._, Vol. XI. 1875.

(23) A. Krohn. "Ueber die Gattung Doliolum u. ihre Arten." _Archiv f. Naturgeschichte_, Bd. XVIII. 1852.

(24) A. Krohn. "Ueber die Entwicklung d. Ascidien." _Müller's Archiv_, 1852.

(25) A. Krohn. "Ueber die Fortpflanzungsverhältnisse d. Botrylliden." _Archiv f. Naturgeschichte_, Vol. XXXV. 1869.

(26) A. Krohn. "Ueber die früheste Bildung d. Botryllenstöcke." _Archiv f. Naturgeschichte_, Vol. XXXV. 1869.

(27) C. Kupffer. "Die Stammverwandschaft zwischen Ascidien u. Wirbelthieren." _Archiv f. mikr. Anat._, Vol. VI. 1870.

(28) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." _Archiv f. mikr. Anat._, Vol. VIII. 1872.

(29) H. Lacaze Duthiers. "Recherches sur l'organisation et l'Embryogénie des Ascidies (Molgula tubulosa)." _Comptes rendus_, May 30, 1870, p. 1154.

(30) H. Lacaze Duthiers. "Les Ascidies simples des Côtes de France" (Development of Molgula). _Archiv Zool. expér._, Vol. III. 1874.

(31) R. Leuckart. "Salpa u. Verwandte." _Zoologische Untersuchungen_, Heft II.

(32) E. Metschnikoff. "Observations sur le développement de quelques animaux (Botryllus and Simple Ascidians)." _Bull. d. l'Acad. Pétersbourg_, Vol. XIII. 1869.

(33) H. Milne-Edwards. "Observations s. l. Ascidies composées des côtes de la Manche." _Mémoires d. l'Institut_, T. XVIII. 1842.

(34) W. Salensky. "Ueber d. embryonale Entwicklungsgeschichte der Salpen." _Zeit. f. wiss. Zool._, B. XXVII. 1877.

(35) W. Salensky. "Ueber die Knospung d. Salpen." _Morphol. Jahrbuch_, Bd. III. 1877.

(36) W. Salensky. "Ueber die Entwicklung d. Hoden u. über den Generationswechsel d. Salpen." _Zeit. f. wiss. Zool._, Bd. XXX. Suppl. 1878.

(37) C. Semper. "Ueber die Entstehung d. geschichteten Cellulose-Epidermis d. Ascidien." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875.

(38) Fr. Todaro. _Sopra lo sviluppo e l'anatomia delle Salpe._ Roma, 1875.

(39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe." _Reale Accademia dei Lincei_, Vol. IV. 1880.