CASTLE. — CELL LINEAGE OP THE ASCIDIAN EGG. 209 



stage. Before the division (Figs. 3 and 4), it will be noted, the cells 

 of the ectodermal half were so arranged that twelve of them were 

 marginal and in contact with cells of the eudodermal half, while four 

 (A*'-^ B*'\ C'S and D'^'S Fig. 3) were entirely surrounded by cells of 

 the ectodermal half. These four now divide along planes perpendicu- 

 lar to the sagittal plane, so that both the daughter cells of each still 

 abut on the median plane. All the marginal cells, however, except 

 the small posterior pair, which remains quiescent, divide in such a way 

 that only one of the daughter cells of each is still marginal and in 

 contact with cells of the endodermal half (Fig. 5, A^-*, A^-^ A''-^, D'^\ 

 D'''^, and the corresponding cells in the right half of the figure). 



The nest division again involves those cells of the ectodermal half 

 of the egg which are marginal, including this time the small posterior 

 cells C^-^ and D^*^, which have lagged one generation behind the other 

 cells of the ectodermal hemisphere, but now give off toward the 

 median plane each a peculiar flattened cell of minute dimensions, un- 

 doubtedly the '' petites cellules cuneiforraes" of Van Beneden et Julin. 

 This division also passes through two other cells of the ectodermal 

 half, C"'' and D"-'', and six anterior and marginal cells of the endo- 

 dermal half, namely, a®-^, a^*'', d®"^ and their mates b^*®, h^-'', and c®**. 



Concomitantly with this division of marginal cells, and to a greater 

 or less extent in consequence of it, the endodermal face of the egg 

 becomes flat, its cells being crowded together at their superficial ends 

 and expanding club-shaped beneath the surface. The ectodermal face 

 at the same time becomes more convex, its cells growing thinner and 

 spreading out so as to cover a greater area. The marginal cells gradu- 

 ally extend around on to the endodermal face, and thus gastrulation 

 begins. 



At this stage (66-cell) the ectodermal half of the egg consists of 

 forty-four cells, of which twenty-four, the more marginal ones, are in 

 the eighth generation, and the remaining twenty, centrally or pos- 

 teriorly situated, are in the seventh generation. 



The endodermal half of the egg, on account of its less rapid cleav- 

 age, is composed of fewer cells, viz. twenty-two, just half the number 

 in the ectodermal portion. Of these, twelve — lying anterior and 

 marginal — are in the seventh generation, and ten — central or pos- 

 terior — in the sixth generation. 



It will thus be seen (1) that the cells of the dorsal half have lagged 

 just one generation behind those of the ventral half, and (2) that cell 

 division has been more active in the anterior and lateral marginal 

 portions of the egg. As a consequence of the first mentioned fact, a 



VOL. XXX. (n. 8. XXII.) 14 



