THE BIOLOGY OF THE CELL SURFACE 



during the previous development of an ^gg are restored to 

 the cytoplasm at this time, that is, to the cytoplasm of 

 the new egg. However, the demonstration that pluri- 

 potency is present gives no evidence of the time when it 

 arose. Merogony may be only an indicator of an earlier 

 established pluripotency. Having no other criterion than 

 fertilizability for the demonstration of pluripotency, we 

 can not exactly define the moment when the egg becomes 

 pluripotent. 



An attractive hypothesis is that pluripotency arises with 

 breakdown of the germinal vesicle when there escapes into 

 the egg-cytoplasm residual nuclear stuff greatly in excess 

 of what remains to form the mitotic complex of the first 

 maturation-division. The potencies might be regarded as 

 identical to, or associated with, this extra-nuclear material. 

 According to this suggestion, the rise of pluripotency would 

 be separated from the fertilizable period, for the fertiliz- 

 ability does not depend upon a particular stage in matura- 

 tion. But since many species of eggs are fertilizable whilst 

 their germinal vesicles are intact, not a single one of these 

 should be pluripotent if we are to relate the rise of pluri- 

 potency to substances escaping into the cytoplasm after 

 breakdown of the germinal vesicle. Where, however, frag- 

 ments taken' from such eggs develop when fertilized, it 

 must be ascertained that the fragments are devoid of stuff 

 having escaped from the germinal vesicle. The lack of 

 data on merogony in such eggs does not warrant a decisive 

 conclusion in these questions. 



There remains another possibility. After the germ-cells 

 have become differentiated from somatic cells through the 

 loss to their nuclei of all potencies, with only the potencies 

 for germ-cells left free in their cytoplasm, they become 

 isolated from the soma. This isolation brings about the 

 escape of all potencies that were up to that time bound in 

 the nuclei, into the cytoplasm. Thus the eggs would 



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