CLE A FACE AND DIFFERENTIATION 



other blastomeres — as happens in the experiment of sep- 

 arating the blastomeres — do they develop separately. In 

 the intact cleaving egg the ectoplasm interacts with that 

 of the neighboring blastomeres and thus insures the normal 

 development into one embryo. 



Secondly, in normal development the egg cleaves into 

 blastomeres, that is, new cell-partitions arise. When exper- 

 imentally eggs are induced to differentiate without cleavage, 

 abnormal development results. In such cases of differ- 

 entiation without cleavage there is always an abnormal 

 accumulation of ectoplasm. 



In the third place: the embryonic axis in eggs of radiate 

 animals and the plane of bilateral symmetry of the 

 embryo in eggs which develop into bilateral animals, arise 

 in the surface and not within the bulk of the egg-substance. 

 If there be gradients of development these are ectoplasmic 

 and not axial, polar or otherwise. 



Fourthly, the ordering and shifting of cytoplasmic 

 inclusions, in terms of primary causes, depend upon the 

 ectoplasm, since the changes in the ground-substance, 

 responsible for these movements, are themselves dependent 

 upon activity in the superficial cytoplasm, as was shown 

 in the chapter on cell-division. 



The importance of the ectoplasm as a causal factor in 

 differentiation is in, addition predicated upon its general 

 properties, its role in the exchange of water between egg 

 and outside world, its necessity for fertilization, its signif- 

 icance in parthenogenesis, its part in initiating division of 

 the cell. These attributes alone would suffice to establish 

 the ectoplasm as a leading factor in differentiation. But 

 there are other and more general considerations. 



The establishment of new surfaces during cleavage alters 

 the physical properties of the protoplasm. With the 

 separation of the egg-substance into blastomeres the origi- 

 nal physical state of the uncleaved egg is altered since some 



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