254 



GENERAL EMBRYOLOGY 



pointed out, cleavage may be irregular as well as indeterminate. 

 And in many of the groups named above, normal development 



FIG. 121. Cleavage in Ascaris megalocephala bivalens. From Jenkinson, after 

 Boveri. 1. Division of the two-cell stage. Elimination of chromatin in the 

 somatic cell Si(AB). la. Chromosomes of the cell S\(AB). 2. Four-cell stage 

 (T-form). In A and B can be seen the eliminated chromatin. The cell Pi has 

 divided into a somatic cell Sz(EMSt), in the descendants of which chromatin 

 elimination occurs, and the cell Pa. 3. Four-cell stage (lozenge-form). A is 

 anterior, A and B, dorsal. 4. Continued chromatin elimination in somatic 

 cells. p2 has divided into Ps, and Ss(C) secondary ectoderm, a, b, primary 

 ectoderm of right side, , 0, of left side. 5. The endoderm cell has been formed 

 and has divided (Ei, Ez). Ps has divided into P-i, the primordial germ cell, and 

 S4(d), tertiary ectoderm. 6. Ventral view at the beginning of invagination. 

 Elimination of chromatin in S(D). The four endoderm cells (E) beginning to 

 invaginate. On each side two mesoderm cells (M) in which granular chromo- 

 somes may be seen, and two stomodaeal cells (St). 



may occur even though interrupted by the removal of parts, 

 by pressure, etc. 



