54 The Ascidian Half -Embryo. 



the anterior products of the anterior cells (fig. 6 : B 51 , b 5-3 ; figs. 1 

 and 16: A 51 , a 53 ) lie slightly below the other cells ; and. the 

 posterior products of the posterior cells (fig. 6 : C 5-2 , c 54 ; figs 

 10 and 16: D 52 , d 54 ) lie slightly above the median products. 

 On a comparison of fig. 6 and fig. 2, it will be seen, however, that 

 the topographical relations of the cells of the fragment are quite 

 different from the normal. For example in fig. 6, the cell c 54 is 

 in contact with B 51 and b 53 , while in the normal egg it lies at the 

 other end of the embryo. A similar rearrangement is still better 

 shown in fig. 10, that of a right T 8 g- embryo, where D 5-2 is in con- 

 tact with A 5,1 , while d 54 is in contact with A 5 - 1 and a 53 . This 

 rearrangement is obviously rendered possible by the absence of 

 the other half of the embryo, so that the cells cohere in a spherical 

 form just as a corresponding number of soap-bubbles. It cannot 

 be considered as a " gliding," for the spindle-axes are from the 

 first accommodated to the changed conditions. That is (figs. 15, 16), 

 the anterior end of the anterior spindles, and the posterior ends 

 of the posterior mitotic figures are swung somewhat toward the 

 original first cleavage-plane of the embryo. 



Chabry's fig. 113 leaves no doubt that the T 8 g- embi^yo of A sci- 

 diella is precisely the same as that described above for Molgula. 

 From Driesch's fig. 5, there is no doubt that in Phallusia the 

 eight cells are arranged as the normal 8 cells. 



i 8 e-— |-f. When activity is again resumed, only the four lower 

 cells are affected, while the dorsal cells remain quiescent. A 12- 

 celled fragment results (figs. 7 and 11), which is exactly equivalent 

 to a half of the normal 24-cell stage (fig. 3). The quiescence of the 

 dorsal cells during the division of the ventral cells is the first in- 

 dication of the alternation of activity in the rhythm of cleavage, 

 which was found to be characteristic of this type of segmentation. 

 As in the preceding stage, when the resting condition is assumed, 

 there is a passive rearrangement of the cells. For example, the 

 cells A 63 and A 64 were segmented along an axis inclined at an 

 angle of 45° to the axis joining their centres at the resting stage. 

 Again the cells D 63 and D 6 - 4 have retreated around the posterior 

 end of the fragment. 



-|-f-. While the eight cells of the lower hemisphere are resting, 

 the four dorsal cells likewise pass into the sixth generation, and a 

 ■Irf stage results (figs. 8, 12). Its resemblance to the half of a 

 normaL 32-cell stage is still less marked than that of a ^-| embiyo 



