MORPHOCHORETIC PATTERN CLEAVAGE 327 



r% 



/ 



B •■'""•■■ 



^•w^"iS'j'->., 





Fig. 18. Coincidence of the first cleavage with the plane of symmetry in the egg of 

 Saccoglossus kow. (enteropneust). (a) Nile blue vital staining of the right 1/2 blastomere; 

 (b) to (f) further cleavage, gastrulation, ventral view of the embryo. A, B, Nile blue staining 

 of the 2/4 blastomeres ; C, D, further cleavage and gastrulation ; E, F, dorso-lateral and 

 ventral view of the larva. From Colwin and Colwin, 1951. 



relative to the plane of symmetry is as variable as in amphibians, but at the 8-cell 

 stage most of the eggs, if they are not of the tetrahedral type, appear as a slightly 

 oblong cupule in which the four dorsal smaller cells (which divide immediately) 

 contain most of the embryonic material. The four larger cells, which divide more 

 slowly, will become Uiiearaphobla^t^ndUh^^ndop^^ (p. 319) : the cytochemical 

 activities make the distinction quite clear^^cuT^Figs. 17, 20-22). In other mammals 

 such a differential cleavage probably also exists, but the disposition of the cells 

 in a morula makes the distinction less schematic^ Among invertebrates, the sea 

 urchin eggs show their micromeres at the antipole, just where the experimental 

 analysis locates the more powerful "vegetalizing" activities. In the enteropneusts 

 (Colwin and Colwin, 1951), a constant coincidence between the first plane of 

 cleavage and the plane of symmetry is again encountered (Fig. 18), as clear as 

 in the protochordates. In the numerous worms, moUusks and related groups 

 commonly refered to as Spiralia, the successive spindles receive a definite location 

 and orientation which, by a special gearing of the processes, results in the acqui- 

 sition of a relatively bilateral cell-distribution. 



It is currently admitted that polarity corresponds to yolk accumulation towards 

 the antipole, and that this mechanical factor plays a role in the orientation of the 



' This is the case for the rabbit egg, in which the careful inquiry of De Geeter (1954) 

 could nevertheless recognize a cytoplasmic organization conforming to the above account. 

 His conclusion finds support in Seidel's recent experiments (1956). The suppression of 

 one 1/2 rabbit blastomere, followed by re-implantation, has been ascertained to yield two 

 types of results. In one type, regulation takes place; in the other, a blastocyst deprived 

 of embryonic knot or primitive streak is obtained. The explanation is that both blastomeres 

 are not equipotent. Only one of them contains the embryonic area of the egg, in full 

 agreement with the cytochemical data concerning the rat (see Figs. 17 and 20). 



Literalure p. 483 



