CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 249 



terior (J^-^ Fig. 65; B"', Fig. 64; I)'-'', Z)^-", Fig. 63; together with the 

 corresponding cells iu the left halves of these Figures) pox'tions. The cyto- 

 plasm is finely granular and pretty homogeneous throughout, except in 

 those portions of the cell most remote from the nucleus, where a certain 

 amount of yolk is to be seen either unassimilated (Fig. 66, O^'^) or in 

 process of assimilation (Figs. 68-70, A^-^^, A^-'^^, A^-^, A^-\ and their mates 

 in quadrant B). In C'-^ (the mate of D''^ in Fig. 63) both conditions 

 are realized. Ai-ound the nucleus is the finely granular protoplasm, 

 and iu those portions of the cell most remote from the nucleus is the 

 unattacked yolk. Forming a sharp line of boundary between the two 

 is a zone in which assimilation is progressing, the yolk granules appear- 

 ing here as large dai'k bodies. The color which the cells of the neuro- 

 muscular ring assume is not so deep a blue as that of the mesenchyme 

 cells ; it is of a more grayish tint. 



B. Later Stages op Gastrulation. 

 (a) From the 112-cell to the 128 -cell Stage. 



Figure 72 (Plate XL) i-epresents a dorsal view of a stage a little more 

 advanced than the 112-cell stage shown in Figure 71. Sections (not 

 figured) of this stage show (cf. sections of an older stage, Figs. 73-77) 

 that the endoderm cells are in mitosis, the spindles being in all cases 

 situated in an approximately horizontal position, so that after division 

 the daughter cells will lie iu a single layer forming a curved plate. The 

 spindles are directed longitudinally in all the cells except two, viz. 

 c^-^ and d^-'^ (cf. Plate X. Fig. 62), in which they lie transverse to the 

 long axis of the embryo. 



Among the mesenchyme cells division has occurred in D'-^, C'^ (cf. 

 Figs. 71 and 74), the spindles standing vertically, as in the case of 

 ji.6^ ^7.6 (Plate X. Figs. 62 and G7), which divided earlier. Vertical 

 spindles are also present in c'-'^'^, d"'-^- (cf. Plate XL Figs. 71 and 75). 



The chorda cells are in the same generation as at the last stage, but 

 the anterior ones are laterally compressed into a flattened or wedge shape, 

 their thinner edges being directed backward. They are situated at the 

 anterior margin of the blastopore (Fig. 72). 



In the neuro-muscular band, two cells on each side of the blastopore 

 (D^-'', D^-^, C^-'', C^-^) are seen to be in mitosis, their spindles being 

 directed toward the centre of the blastopore. Xo evidence of division 

 can be seen in any other cells of the embryo. It is therefore clear that 

 the considerable advance in the process of gastrulation which is seen to 



VOL. XXVII. — NO. 7, 4 



