castle: embryology of ciona intestinalis. 237 



paper, it necessitates modification of several minor statements, as will be 

 indicated in detail later. 



The 24-cell stage, it has been seen, arises from the 16-cell stage by an 

 earlier division on the part of the cells of the ventral hemisphere than 

 occurs in those of the dorsal hemisphere. Accordingly, we find that at 

 the 24-cell stage the ventral hemisphere consists of sixteen cells, whereas 

 the dorsal hemisphere is made up of only eight. These eight are com- 

 pressed into a columnar form by the overgrowth of the cells of the 

 ventral hemisphere already begun. (See Plate VII. Fig. 44.) Their nu- 

 clei lie in a superficial position, while their deep ends are heavily laden 

 with unassimilated yolk. They retain this columnar form up to and 

 throughout gastrulation. In number, they are soon brought up to an 

 eqiiality with the cells of the ventral hemisphere by division, which 

 leads to the 32-cell stage (Plate IX. Figs. 53 and 54) and places all the 

 cells of the egg in the sixth generation. 



Pi'esently the cells of the ventral hemisphere again anticipate in 

 division those of the dorsal hemisphere, this time by a still longer inter- 

 val. Among the cells of the venti-al hemisphere differences in the time 

 of division could, as we have seen, be detected at the preceding cleavage. 

 At the present cleavage the differences become more pronounced. In 

 particular, the small posterior cells, C^-^, D^-^ (Plate IX. Figs. 53 and 54), 

 divide enough later than their fellows to allow us to recognize a 46-cell 

 stage (Plate IX. Figs. 55 and 56), made up as follows : — 



Ventral hemisphere, 28 cells in the seventh generation, 



2 cells (6^^-^, B^-^) in the sixth generation. 

 Dorsal hemisphere, 16 cells in the sixth genex'ation. 



46 



When the two small cells C'^ />®•^ divide, which they do earlier than 

 the cells of the dorsal hemisphere, a stage of forty-eight cells is reached, 

 all the cells of the ventral hemisphere (thirty-two in number) being in 

 the seventh generation, and those of the dorsal hemisphere (sixteen in 

 number) being in the sixth generation. Such a stage is shown in Plate X. 

 Figs. 57 and 58. 



(6) 48-cell Stage. 



The embryo shown in Figures 57 and 58 has a vertical axis the length 

 of which is equal to that of its antero-posterior axis, if not greater. 

 Accordingly it has been found easier to maintain this axis in a horizon- 

 tal position, and hence more convenient to represent the egg as viewed 



