CLEAVAGE AND DIFFERENTIATION 



blastomeres other than those that normally hold them. 

 For the cleavage planes bear any relation to the axis of 

 stratification. 



Even more significant is the second result, namely, that 

 fragments of clear inclusion-free cytoplasm taken from 

 centrifuged eggs, whether these eggs cleave determinately 

 or indeterminately, develop when fertilized as whole eggs. 

 Many unfertilized eggs — e.g., those of sea-urchins and those 

 of other species whose germinal vesicles have broken down 

 — can be strongly deformed by pressure, centrifugal force, 

 etc. This is due to the fact that the fluid protoplasm is 

 enclosed by a highly elastic membrane. Such eggs when 

 centrifuged are pulled out into strands, a fact known already 

 many years ago. With a greater degree of centrifuging 

 the strands break up into fragments. Some of these frag- 

 ments, with or without the egg nucleus, are composed of 

 the clear hyaline cytoplasm only. Such clear fragments 

 devoid of inclusions are capable of fertilization and develop- 

 ment; they cleave as whole eggs.-^ 



These two experimental findings alone render untenable 

 the theory that considers the visible inclusions as organ- 

 forming substances. The differentiation from the single 

 cell, the Qgg, to the complex multi-cellular embryo can not 

 be related to the distribution of visible materials suspended 

 in the cytoplasm. That these materials have functions, 

 no one would deny. Most of them, oil and yolk, are food 

 for the embryo or larva; the remainder are doubtless for 

 respiration, secretion, etc. They move with the cyto- 

 plasmic ebb and flow but are not themselves the tide of life. 



The orderly progressive shifting of the visible multi- 

 colored spherules and granules in a living egg, especially 

 in one with determinate cleavage, is an entrancing spec- 

 tacle: in defiance to gravity, oil, yolk, mitchondria and 



1 Lillie, F. R., igo6. 



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