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 
equality 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. 
Presently 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 ventral 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, C59, D** (Plate IX. Figs. 53 and 54), 
divide enough later than their fellows to allow us to recognise a 46-cell 
Stage (Plate IX. Figs. 55 and 56), made up as follows : — 
Ventral hemisphere, 28 cells in the seventh generation, 
2 cells (C5, D**5) in the sixth generation. 
Dorsal hemisphere, 16 cells in the sixth generation. 
46 
When the two small cells O**, *9. divide, which they do earlier than 
the cells of the dorsal hemisphere, a stage of forty-eight cells is reathed, 
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. 
(P) 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 
