castle: embryology of ciona intestinalis. 245 



Dorsal hemisphere : — 



2 mesoderm cells in the 7th generation. 

 10 chorda " 7th 



10 endoderm " Gth " 



22 — 



90 (in ventral hemisphere). 

 TT2 

 The process of gastrulation has at this stage already set in. Nut only 

 is the ectoderm growing over so as to envelop the dorsal hemisphere, 

 but the latter is at the same time sinking down and becoming saucer- 

 shaped. (Cf. Figs. 66 and 77.) Accordingly, gastrulation may lie said 

 to take place by a combination of the two processes of epiboly and 

 invagination. 



(b) Differentiation of the Principal Organs as seen at the 11^-cell 



Stage, 

 a. Topographical. 



We will now consider this same embryo (Plate XI. Fig. 71) with 

 reference to the ultimate fate of its cells. At the depressed centre of 

 its dorsal surface, surrounding the point of formation of the polar glob- 

 ules, we find the ten cells of the definitive endoderm, all in the sixth 

 generation and containing each a very large nucleus. They are a 66 , « 6-8 , 

 d 65 , d 6,s , d 6J , and the corresponding cells in quadrants B and 0. Two 

 of them are derived from each of the anterior quadrants (A and B), and 

 three from each of the posterior quadrants (Cand D). Together they 

 constitute the entire fundament of the definitive larval endoderm. 



The endoderm fundament is surrounded by two concentric rows of 

 cells from which are derived some of the most important organs of the 

 larva. The inner row or ring of cells we will call the chorda-mesenchgme 

 ring, because it is destined to produce the chorda and mesenchyme. In 

 it we must include the small flattened cells, C 7,5 , D" : \ but not their sis- 

 ter cells, C 7 - 6 , .D 7 - 6 , which, though in contact superficially with endoderm 

 cells, really belong, as their fate shows, in the second or outer ring. 



The chorda mother cells, all of which are included in the chorda- 

 mesenchyme ring, are derived, as has been already stated, in part from 

 the anterior and in part from the posterior quadrants. Those derived 

 from the anterior quadrants are at this stage eight in number. They 

 form the anterior segment of the chorda-mesenchynio ring (Fig. 71, 

 a 7 - 9 , a 7-10 , a 7-13 , a 7-14 , and the corresponding cells on the left of the 

 median plane). The posterior chorda cells are only two in number 



