174 



VERTEBRATA 



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 "il a C D 



6 ^BSx *v ^RBv V^Blltthv y. 







FIG. 1. Early stages of Ascidia and Amphioxus. a, blastopore ; &, neural 

 groove; c, neural folds; d, closed portion of neural tube; e, commencing 

 oral invagination (stomodamm) of Ascidian tadpole ; /, right ami left cloaca! 

 imaginations of Ascidian tadpole ; g, anterior opening of neural tube of 

 Amphioxus, coincident with the later developed olfactory pit ; h, wall of jjne 

 of the series of paired outgrowths of archenteron or ccelomic pouches of 

 Amphioxus ; i, ectoderm ; fc, endoderm (of diblastula phase) ; /, notochord, 

 derived from endoderm ; m, 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 ; 7, lumen of ccelomic 

 pouches of Amphioxus, once continuous with m, but nipped off in the course 

 of development. 



A, B, C, D. Four stages in development of Asciilia, 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. DD. 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. H. 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 coalomic pouches or 

 inesoblastic somites q. L. Transverse section of a much further advanced 

 embryo of Amphioxus, showing nerve-tube, notochord, and gut ; the walls 

 of the ccelomic pouches are now converted into muscular tissue and the 

 pouch cavity (7) compressed, (All the figures after Kowalewsky, 77, 18.) 



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 



FIG. 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 myclonic 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 MJ^*J*J 

 but it required the intermediate proposition by Balfour of as ' y l e " 

 a group Chordata, to comprise the two divisions Tunicata brates. 

 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 Artkropoda, are not arthropodous ; and 

 many Echinoderma 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 innovation. s ' "* 

 The remarkable marine worm Balanoglossm originally 

 described by Delia Chiaje at the end of the 1 8th century j n y e rt 

 was shown in 1866 by Kowalewsky (j) to possess a series brate 

 of pharyngeal gill-slits similar to those of Tunicata andP n >" lun: 

 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 



