CASTLE : EMBKYOLOGY OF CIONA INTESTINALIS. 251 



nearer to the superficial thau to the deep euds of the cells. The most 

 anterior mesenchyme cells appear in Figure 77 {A^-^% A^-^^, B^'^% B^'^^). 

 They are descended from A''-^, B"'-^ (Plate X. Fig. 62), in which cells the 

 division leading to the eighth generation occurred at a stage considerably 

 earlier than this. (See Plate X. Fig. 67.) In this case also {A~-^, B'-^) 

 the spindles stood vertically and division was unequal, but the ?nore 

 superficial daughter cell was the larger, the deeper one being small and 

 almost entirely filled with unassimilated yolk. (See Plate XL P"'ig. 77, 



^8.12^ ^8.11^ ^8.12^ ^8.n ■) 



The jiosterior chorda cells (o?'"", Fig. 77, and c'^-" in the left half of 

 Fig. 7G, not lettered) show no essential change since the last stage figured 

 (cf. Plate X. Figs. 66 and 67). The anterior chorda cells do not appear 

 in the sections figured ; they are still in the seventh generation. 



Division has been completed in D^-'', D^'^, C^-'', C^-^ (Fig. 72), four of 

 the neuro-muscular cells lying at the margin of the blastopore. (See 

 Figs. 7i-76, D^-^% I)^-^*, n^-'^^, D^-^% and the corresponding cells in 

 quadrant C.) The other cells of the neuro-muscular ring and the entire 

 ectodermal group have not been essentially modified since the last stage 

 figured (Fig. 72). 



If the foregoing account is correct, the embryo, sections of which are 

 shown in Figures 73-77, contains one hundred and twenty-eight cells, 

 distributed as follows. 



Ventral hemisphere : — 



Ectodermal group : 



( ectoderm. 



64 cells in the 9th generation -x ,, 



° ( nerve cells. 



Equatorial band : 



2 ectoderm cells in the 9th generation. 



8 neuro-muscular cells in the 9th generation. 



10 " " " 8th " 



2 " " " 7th " 



8 mesenchyme " 8th " 



2 mesenchyme " 7th " 



32 — 



96 



Dorsal hemisphere : — 



2 mesenchj'me cells in the 7th generation. 

 10 chorda " " " 



20 endoderm " " " 



32 



128 



