360 EDWIN G. CONKLIN 



into strands oi' fibers which are not to be regarded as preformed 

 structures. It is clear that the localization of yolk at the vegetal 

 pole in normal eggs and its return to that pole after it has been 

 displaced is due to some differentiation of the spongioplasm at 

 the two poles, for otherwise the yolk might be localized at any 

 pole and would remain wherever it happened to be thrown. In 

 normal eggs the spongioplasm is more abundant at the animal 

 pole than elsewhere, but in centrifuged eggs it may be less abun- 

 dant at this pole and yet normal conditions may be restored after 

 centrifuging. It is therefore necessary to assume that the 

 spongioplasm differs in some way, perhaps in elasticity or vis- 

 cosity, at the two poles. If this material is more elastic or more 

 contractile in the region of the animal pole than elsewhere in the 

 cell the localization of cytoplasm and yolk in normal eggs and the 

 return of dislocated substances to their normal positions would 

 find an explanation. Furthermore the connections of centro- 

 spheres and nuclei, the orientations of mitotic figures and the 

 progressive localizations of cell substances indicate that the 

 spongioplasm must differ in different regions of the egg and at 

 different stages of development. 



The question may well be raised whether the spongioplasm, or 

 more viscid portion of the protoplasm, is not the real formative 

 material, while the cytolymph as well as the oil and yolk are mere 

 inclusions. I have already indicated that it is the more im- 

 portant or indispensable part of the protoplasm, as is shown by 

 experiments with centrifugal force and also by the desiccation of 

 protoplasm in seeds and in certain animals (rotifers, tardigrades, 

 etc.) ; in both of these cases the fluid within the protoplasm may 

 be largely eliminated without permanently destroying or injur- 

 ing the protoplasm. It is well known that the fluidity or vis- 

 cosity of protoplasm depends upon its water content and that 

 this differs under different external conditions and in different 

 stages of the cell cycle. Evidently the colloids in protoplasm 

 may change from gels to sols and vice versa. It does not seem 

 wise therefore to identify as protoplasm the gels only. The most 

 convincing work which has yet been done on the physical prop- 

 erties of protoplasm in living cells is that of Kite, Chambers, 



