CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 243 
all in the sixth generation. The first two zones are descended from the 
four ventral cells of the 8-cell stage, i. e. from the four cells most remote 
from the point of formation of the polar globules. The third zone is 
descended from the four dorsal cells of the 8-cell stage. The ectoderm 
is derived chiefly from the first zone, — that is, the zone encircling, the 
animal pole; — the mesoderm is derived chiefly from the second zone, 
and the endoderm exclusively from the third zone. 
This zonal arrangement persists throughout cleavage and the early 
Stages of gastrulation, but its symmetry is at each succeeding stage dis- 
turbed to an inereasing extent by the fact that cell division is less rapid 
at the posterior than at the anterior end of the embryo. 
9. Although, as just stated, cleavage progresses with unequal rapidity 
at the two poles of the antero-posterior axis, as well as at those of the 
dorso-ventral axis, it is equal in rate at the two poles of the third axis 
of the egg, viz. the transverse. The last mentioned fact serves to main- 
tain the perfectly bilateral form of the embryo. 
The differentiation of the poles of the dorso-ventral and antero-poste- 
rior axes, the reader will recall, was already recognizable by structural 
oytoplasmie differences in the unsegmented ovum. The form and rate of 
cleavage are therefore manifestly predetermined by the internal constitution 
of the ovum. 
3. Gastrulation. 
A. EARLY STAGES OF GASTRULATION. 
(a) 112-cell Stage. 
An embryo a little more advanced in development than the one 
represented in Figures 61 and 62 (Plate X.) is shown in dorsal view 
in Figure 71 (Plate XL). No new divisions have occurred in the 
dorsal hemisphere, which accordingly consists, as at the last stage, of 
twenty-two cells. In the equatorial band, the four cells which were 
preparing for division at the 76-cell stage (Plate X. Fig. 62, D, ou. 
A™® and 575) are seen in Figure 71 to have divided, though in the case 
of A" and B^, on account of the vertical position of the spindles (ef. 
Fig. 67), only the more superficial daughter cell is in each instance visi- 
blo (49%, Be, Fig. 71). No further divisions have occurred in the 
equatorial band, whieh therefore consists at this stage of twenty-six cells, 
all in the eighth generation except the group of six cells arranged in 
crescent form at the posterior end of the embryo, viz. D" 5, Ds, 7-9, and 
the corresponding cells in quadrant C. These cells have lingered in the 
seventh generation later than all other cells of the ventral hemisphere. 
