CASTLE: EMBRYOLOGY OF CIOX.V [NTESTINALIS. 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 itssymmetry is at each succeeding stage dis- 

 turbed to an increasing extent by the fact that cell division is less rapid 

 at the posterior than at the anterior end of the embryo. 



2. 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 

 cytoplasmic differences in the unsegmented ovum. Tit e form and rate of 

 cleavage are therefore manifestly predetermined by the internal constitution 

 of the ovum. 



3. Gastrulation. 



A. Early Stages op 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 XI.). Xo 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"~, C 7,7 , 

 J 7 - 6 , and IP- 6 ) are seen in Figure 71 to have divided, though in the case 

 of A 7 - 6 and B"'- G , on account of the vertical position of the spindles (cf. 

 Fig. 67), only the more superficial daughter cell is in each instance visi- 

 ble (A s - 12 , B S12 , Fig. 71). Xo further divisions have occurred in the 

 equatorial band, which 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. Z) 7,5 , Z) 7 - 6 , Z> : ", and 

 the corresponding cells in quadrant C. These cells have lingered in the 

 seventh generation later than all other cells of the ventral hemisphere. 



