ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
57 
196 and 197), which represent ventral and dorsal views of one and the same egg, 
the cells at both poles are seen to be in process of division, and the only cells in 
the entire embryo which are not dividing are the small posterior cells (B). The 
cells of the dorsal hemisphere are in the late anaphase or telophase, and their nuclei 
are still small and densely chromatic; the cells of the ventral hemisphere are all in 
the equatorial plate stage. These figures show most conclusively that all the cells of 
the embryo divide during this sixth cleavage and are advanced from the sixth to 
the seventh generation, and they therefore make impossible Castle's view that the 
cells of the dorsal hemisphere remain undivided, while those of the ventral hemt- 
sphere divide twice. Another evidence that the cells which are shown dividing in 
his figures 57 and 58 are not the same ones which have just divided in his figure 55 
may be found in the fact that but sixteen of these cells are shown dividing in the 
former figures, whereas the other sixteen cells, which, according to Castle, belong 
to the ventral hemisphere, are in the resting stage, exactly as are the sixteen cells 
which immediately surround the dorsal pole; at the two previous cleavages, and as 
I have found also, at the two subsequent ones, all the cells of the ventral hemisphere 
divide simultaneously, and this fact speaks against Castle's view that at the 48-cell 
stage one-half of these cells divides and the other half does not. 
Since the dorsal hemisphere, shown in his figure 55, contains twenty-eight cells 
of the seventh generation and two of the sixth, while the ventral hemisphere shown 
in figure 56 contains only sixteen cells of the sixth generation, it is evident that if 
the egg is inverted in its orientation at this stage the equator must be shifted nearer 
to the dorsal hemisphere so as to reduce the number of dorsal cells to sixteen and 
to increase the number of ventral cells to thirty, or, after the division of the two 
small posterior cells, to thirty-two. This is just what Castle has done; in his 
description of the 48-cell stage (pp. 238, 239) he says that at this stage the embryo 
is composed of three zones of sixteen cells each, as follows : 
Ventral hemisphere 
16 cells of the seventh generation, ectodermal group. 
16 cells of the seventh generation, equatorial band. 
Dorsal hemisphere 
16 cells of the sixth generation, endoderm, chorda and mesoderm. 
48 cells. | 
Immediately after this stage the 64-cell stage is reached by the division of the 
sixteen cells of the ectodermal group. Castle has tabulated the cells of this stage 
as follows: 
_ Ventral hemisphere 
2 82 cells in the eighth generation, ectodermal group. 
16 celis i in vse seventh generation, the equatorial band. 
m. 48 
Dorsal hemisphere 
16 cells in the sixth generation. 
64 cells. 
_ 8 JOURN. А. N. 8. PHILA., VOL. XIII. 
