ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 57 



L 96 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 6-3 ). 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 

 eel Is of the dorsal hemisphere remain undivided, while those of the ventral hemi- 

 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 he 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 ami 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 hand. 

 Dorsal hemisphere 



16 cells of the sixth generation, endoderm, chorda and mesoderm. 



1 8 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 



32 cells in the eighth generation, ectodermal group. 

 16 cells in the seventh generation, the equatorial band. 



48 

 Dorsal hemisphere 



16 cells in the sixth generation. 

 lil cells. 



8 JOUEN. A. X. S. PHILA., VOL. XIII. 



