CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 241 
Ventral hemisphere (designated by the letters A, B, C, D): 
32 cells in the eighth generation — the ectodermal group. 
12 cells in the“ pS 
10 cells in the seventh  “ 
54— 
Dorsal hemisphere (designated by a, b, c, d): — 
2 mesoderm cells in the seventh generation. 
} = the equatorial band. 
10 chorda cells in the id " 
10 endoderm cells in the sixth di 
22 — 
76 
The ectodermal group of the ventral hemisphere contains the same 
number of cells as at the 64-cell stage, viz. thirty-two, — sixteen on 
each side of the median plane, ten of them being derived from an ante- 
rior quadrant (4), six from a posterior quadrant (D). They cover 
nearly the entire ventral surface of the egg. (See Plate X. Fig. 61, 
Aet 499 489 4810 4918 4814 and Dèt- D*9. as well as the corre- 
sponding cells in the right half of the figure.) All the cells of this group 
are in the cighth generation. 
The equatorial band now contains six more cells than at the 64-cell 
stage, in consequence of the completion of divisions foreshadowed at 
that stage in the cells 47%, AT, B", BY, 07% and D'* (Fig. 60). 
It now consists of twenty-two cells, which, in passing from the posterior 
end forward, are DI DA AT AUC SP DA A, A A a, 
and 4977, with the corresponding cells in quadrants B and C (Fig. 62). 
Six of the cells on each side of the median plane are derived from a 
posterior and five from an anterior quadrant. Signs of approaching di- 
vision have at this stage become visible in four of the cells of this equa- 
torial band, viz. 47:9, BT, D'", and C"". In the case of the first two 
cells mentioned the spindles stand vertically (cf. Plate X. Fig. 67, 47:0) ; 
in the other two cells (077, D'*") the spindles are nearly horizontal in 
position, though their antero-lateral ends lie at a slightly higher level 
than the opposite ends. 
There are only six cells remaining in the equatorial band which nei- 
ther have passed into the eighth generation nor show any signs of imme- 
diately doing so. They are grouped at the posterior end of the embryo, 
which has been repeatedly pointed out as the region of slowest cleavage 
among the cells of the ventral hemisphere. The six cells in question 
are DS, D'5. DTS, and the corresponding cells in quadrant € (Fig. 62). 
