DIFFERENTIATION, HEREDITY, SEX 



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may be subjected, during their early cleavages, to deforming 

 pressure so that the planes of cell division appear in abnormal 

 relations to one another and to the egg as a whole (Hertwig, 

 Born). Thus in the sea-urchin (Driesch) the blastomeres of 

 the eight-cell stage instead of forming a spheroidal group may 

 be forced into the form of a flat plate (Fig. 135). When 

 released from the pressure such eggs form perfectly typical 



FIG. 132. Gastrulae and plutei from isolated blastomeres of the sea-urchins, 

 Echinus (A-D), and Sphcerechinits (E-G). After Driesch. A. Gastrula from 

 entire egg. B. Gastrula from one blastomere of the two-cell stage. C. Gastrula 

 from one blastomere of the four-cell stage. D. Gastrula from one blastomere of 

 the eight-cell stage. E. Normal pluteus. F. Pluteus from one blastomere of 

 the two-cell stage. G. Pluteus from one blastomere of the four-cell stage. 



larvae. And even in a form like the Annulate, Nereis, whose 

 cleavage is determinate and whose blastomeres are highly 

 different iated, Wilson has found that when the egg, subjected 

 to pressure, became divided by vertical planes into a flat plate 

 of eight cells, each one contained substance normally found only 

 in the macromeres of the lower pole; when released these 

 eight cells divided into sixteen, eight micromeres and eight 

 macromeres, instead of into the normal twelve and four 



