Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/199

Rh VERTEBRATA 179 Vertebrate organs of this animal (2). This work alone would not have acquired historic importance, although it is the starting-point of what may be called strict cellular embryology, as compared with the less severely histological works of previous students. But it was accompanied by an account (j) of the development of Ascidia mamillata, one of the so-called Tunicate Molluscs, in which it was demonstrated by Kowalewsky, not only that this supposed Mollusc possesses when first hatched from its egg-envelope a notochord, pharyngeal gill -slits, and a tubular dorsal nerve-cord and brain, but that these three characteristic ally Vertebrate features of organization originate from the same cell-layers of the embryo, and in essentially the same way as in Amphioxus, whilst the cell-layers themselves originate from the egg-cell in the two animals by precisely A 7 B f, C D b FIG. 1. Early stages of Ascidia and Amphioxus. a, blastopore ; 6, neural groove ; c, neural folds ; d, closed portion of neural tube ; e, commencing oral invagination (stomodaeum) of Ascidian tadpole ; /, right and left cloaca! imaginations of Ascidian tadpole; g, anterior opening of neural tube of Amphioxus, coincident with the later developed olfactory pit ; h, wall of one of the series of paired outgrowths of archenteron or coelomic pouches of Amphioxus; i, ectoderm; fc, endoderm (of diblastula phase); I, notochord, derived from endoderm ; ra, cavity of gut ; n, cavity of nerve-tube ; o, wall of nerve-tube, formed by upgrowth and union of neural folds ; p, mesoblast of tail of Ascidian tadpole, derived from endoderm ; q, lumen of ccelomic jiouches of Amphioxus, once continuous with m, but nijiped off in the course of development. A, B, C, D. Four stages in development of Ascidia, surface views showing gradual enclosure of neural area. E, F, G. Three similar stages in develop ment of Amphioxus. AA. Vertical antero-posterior median section of A. BB. Similar section of B. I)D. Similar section of a stage a little earlier than D. EE. Similar section of E (Amphioxus). GG. Similar section of embryo repre sented in G. II. Transverse section of diblastula stage of Amphioxus, with widely open blastopore (earlier than A or E). I. Transverse (right and left) section about the middle of F, showing neural area. K. Transverse section about middle of G, showing nerve-tube, notochord, and crelomic pouches or mesoblastic somites q. L. Transverse section of a much further advanced embryo of Amphinxus, showing nerve-tube, notochord, and put ; the walls of the coelomic pouches are now converted into muscular tissue and the pouch cavity (q) compressed. (All the figures after Kowalewsky, 77, /&amp;lt;?.) similar movements of cell division and invagination (see figs. 1 and 2). Kowalewsky s discoveries established once for all that the Ascidian tadpole is identical in three very special and distinct features of structure with the frog s tadpole. No classification which pretended to set forth the genetic affinities of animals could henceforth separate POSITION HE. SPIRACLE CILLSL1TSI; SPINAL CHORD /, BBAm MOlTB Fio. 2. Diagram illustrating relationship of tadpoles of Frog and Ascidian. The two upper figures represent surface views of the tadpoles ; the two lower ones show in place the chief Vertebrate organs, viz., notochord, gill-slits, nerve-tube, and myelonic eye. (From Lankester s Degeneration.) the Ascidian from the Vertebrata, and with it the Ascidian brought the whole series of Tunicata. The admission of Tunicata as a group of Vertebrata was Admis- proposed by the present writer as long ago as 1877 WiJjJ^ but it required the intermediate proposition by Balfour of Jy^j a group Chordata, to comprise the two divisions Tunicata i, ra tes. and Vertebrata, in order to render the final admission of Tunicata to their proper association with the Vertebrata of Cuvier palatable to systematists. As an objection to the simple inclusion of Tunicata in the great phylum Verte brata it has been urged that Tunicata do not possess verte bra, a proposition which is equally true of Amphioxus and of some Fishes. Shifting the objection, some writers have maintained that the vertebration of the Vertebrata may be understood as having reference to the segmenta tion of the muscles of the body-wall, which is exhibited by all Cuvier s Vertebrata without exception, inclusive of Amphioxus, though not by Tunicata. To this it may be replied that the Ascidian tadpole, and more clearly the free-swimming Tunicate Appendicularia (see fig. 9), do ex hibit a segmentation of the muscles of the hinder part of the body-wall similar to and identical with that of Amphi oxus, whilst no such strict application of a name in its original descriptive sense is desirable in systematic nomen clature. All Gastropoda (it has been pointed out) are not gastropodous ; all Arthropoda are not arthropodous ; and many Echmoderma are not echinodermous. It is, in the present writer s opinion, better to retain an historic and familiar name for the great branch of the animal pedigree to which it has become necessary to admit forms whose affinities therewith were at one time unsuspected rather than to sacrifice historical significance to a futile striving after etymological accuracy. The admission of Tunicata to association with Cuvier s Inclu- Vertebrata has been followed by a further&quot; innovation. &quot; ^ The remarkable marine worm Balanoglossus originally ^&quot;^ described by Delia Chiaje at the end of the 18th century j n v erte was shown in 1866 by Kowalewsky (j) to possess a series brate of pharyngeal gill-slits similar to those of Tunicata and phylum. Amphioxus. Later researches by Bateson (6) have de monstrated that Balanoglossus develops in embryonic life a short notochord, whilst its nerve-cord is, in part at least, tubular, and similar in position and relations to the median