20 • SEX DIFFERENTIATION 



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hus far we have considered development as a cellular phenomenon 

 within the egg. We know, however, from the cross fertilization of two in- 

 dividuals of different morphological characteristics, that the egg develops 

 with the characteristics of both the parents. Since the sperm contributes 

 chiefly nuclear material, it seems clear that the genes of the nucleus react in 

 some way with constitutents in the cytoplasm to produce the final result in 

 the individual. 



One of the first questions which arises concerns what the cytoplasm can 

 do in the way of development without the presence of nuclear genes. This 

 can be answered by an experiment in which the sea urchin egg is centrifuged 

 at very high speeds and the egg is torn into two parts, one containing the 

 nucleus of the egg, the other having no nucleus. In Chapter 4 studies on 

 fertilization showed that excess sodium chloride stimulates the sea urchin 

 egg to develop without a spermatozoon. If now we add sodium chloride to 

 these enucleated eggs, development is stimulated. They undergo cell division 

 without either a sperm or an egg nucleus. It is found that these enucleated 

 eggs will develop through the cleavage stages to the blastula stage but will 

 go no farther. The enucleated eggs will not differentiate. The lack of a 

 nucleus, then, may possibly be correlated with a lack of differentiation. Of 

 course, it could also be true that the nucleus was simply necessary to maintain 



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