CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 249 
terior (DS Fig. 65; Den Fig. 64; D, DUN, Fig. 63; together with the 
corresponding cells in the left halves of these Figures) portions. 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, (9m Sorin 
process of assimilation (Figs. 68-70, 4919: 49.15. 498. 49 and their mates 
in quadrant B). In O75 (the mate of Di? in Fig. 63) both conditions 
are realized. Around the nucleus is the finely granular protoplasm, 
and in 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 dark bodies. The color which tho 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 or GASTRULATION. 
(a) From the 112-cell to the 128-cell Stage. 
Figure 72 (Plate XI.) represents 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 stago, l'igs. 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 ina single layer forming a curved plate. The 
spindles are directed longitudinally in all the cells except two, viz. 
€95 and des (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 DES, 078 (ef, 
Fig 
A’, 179 (Plate X. Figs. 62 and 67), which divided earlier. Vertical 
de 
. T1 and 74), the spindles standing vertically, as in tho case of 
spindles are also present in ce", 47 (cf. Plate XI. 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 nenro-muscular band, two cells on each side of the blastopore 
(DE, DES, 097, 095) aro seen to be in mitosis, their spindles being 
directed toward the centre of the blastopore. No evidence of division 
can be seen in any other cells of tho embryo. Tt is therefore clear that 
the considerable advance in the process o 
gastrulation which is seen to 
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