CASTLE : EMBRYOLOGY OF CIONA INTESTINALIS. 241 



> =: the equatorial band. 



Ventral hemisphere (designated by the letters A, B, C, D): 



32 cells in the eighth generation = the ectodermal group. 

 12 cells in the " 

 10 cells in the seventh ' 

 54 — 



Dorsal hemisphere (designated by a, b, c, d): — 



2 mesoderm cells in the seventh generation. 

 10 chorda cells in the " " 



10 endoderm cells in the sixth " 



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 (A), six from a posterior quadrant (Z)). They cover 

 nearly the entire ventral surface of the egg. (See Plate X. Fig. 61, 

 ^8.i_j8-6^ ^8-9, ^8W A'-'^, A'-'*, and Z>si-i>««, as well as the corre- 

 sponding cells in the right half of the figure.) All the cells of this group 

 are in the eighth 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 A'S A'-^, B''^ B'-\ C^-", and i)^-* (Fig. 60). 

 It now consists of twenty-two cells, which, in passing from the posterior 

 end forward, are D'^ D''-\ D'\ D'-^ D^\ D^\ A''-^, A^-^% A^-^', A^-\ 

 and A^-'', with the corresponding cells in quadrants B and (7 (Fig. 62). 

 Six of the cells on each side of the median plane are derived from a 

 posterior and five from au anterior quadrant. Signs of approaching di- 

 vision have at this stage become visible in four of the cells of this equa- 

 torial band, viz. A''-^, B'^ D^-'', and C^-''. In the case of the first two 

 cells mentioned the spindles stand vertically (cf. Plate X. Fig. 67, A^-^) ; 

 in the other two cells {C''-'', 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 IP-^, W-^, D^-^, and the corresponding cells in quadrant O (Fig. 62). 



