CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 355 



parts is due to a framework of more viscid protoplasm and that 

 the return of displaced parts to normal positions is due in the 

 main to the elasticity or contractility of this framework. The 

 evidence in favor of such a view may be summarized as follows : — 



a. The substances of the egg of Crepidula are never com- 

 pletely stratified by centrifugal force of from 600 to 2000 times 

 gravity. Of all substances in the egg the yolk and cytoplasm 

 are most completely stratified and yet the boundary between the 

 two is never a plane, as it would be if the substances were free 

 to move according to their specific weights, but the boundary 

 between these substances is an irregular one with 'lanes' or 

 projections of cytoplasm into the yolk. This indicates that 

 while there is a relatively large amount of cytoplasm which is 

 freely movable within the cell, there is a small amount of more 

 viscid substance which penetrates every part of the cell and is 

 especially abundant in nuclei, centrospheres and mitotic figures; 

 this viscid material prevents the complete stratification of cell 

 substances according to their specific weights. 



b. In many instances strands of this viscid substance may be 

 seen running through various portions of eggs; such strands are 

 seen most plainly in the mitotic spindles and astral radiations 

 of dividing cells and also in the connections between nuclei and 

 centrospheres and between the latter and the cell surface in 

 dividing cells. These connections may be stretched or bent, 

 but are rarely broken. The fact that when the animal pole 

 of the egg is centrifugal in position the spindle may be stretched 

 or distorted and the surrounding cytoplasm may be forced away, 

 while yolk comes to be densely packed around the spindle, proves 

 that the spindle is not merely the expression of lines of force, 

 like iron filings in a magnetic field, but that it is a relatively 

 persistent structure of a viscid or gelatinous character. The 

 same is true also of resting nuclei and centrospheres and of the 

 strands which connect these to the cell surface (see fig. 60). 



c. This viscid material is most abundant in spindles and 

 astral radiations of dividing cells and in nuclei, centrospheres 

 and the connections between these and the cell surface in rest- 



