The Growth of the Oocyte in Antedon. 



385 



formation. Yolk formation takes place in this area of the cytoplasm exactly 

 as elsewhere. 



The clear area often accompanying the yolk nucleus, usually on its outer 

 surface, is an artifact, and results from the inability of the acid fixing 

 reagent to coagulate the metaplastic substances with which the cytoplasm, 

 and especially the yolk nuclear area of the cytoplasm, is loaded. 



The yolk nucleus, therefore, is simply a region of the cytoplasm on to 

 which waste material discharged from the nucleolus has diffused. The 

 period at which this diffusion occurs, as also all subsequent changes in the 

 form and position of the yolk nucleus, are determined by the progressive 

 change in the physical consistency of the cytoplasm which precedes and 

 accompanies yolk formation. Thus the yolk nucleus, like the nucleolar 

 material discharged subsequent to its formation, takes no part in the cell- 

 processes. 



The, Nucleolus. — The spherical form of the nucleolus, a form in which 

 minimal surface area is associated with maximal bulk, stands in striking 

 contrast to the subdivided condition of the chromatin, and indicates the 

 absence of any reaction between the caryolymph and the nucleolar substance 

 at the surface of the nucleolus ; the latter structure passively growing by the 

 deposition of material on its surface from solution in the caryolymph. 



This nucleolar material consists of two substances : the one acidophile and 

 extending throughout the nucleolus, the other deeply basophile and borne by 

 the acidophile ground substance, to which its presence imparts a considerably 

 firmer consistency. 



Nucleolar activity, which proceeds from within outwards, results in the 

 breakdown of the basophile constituent ; the products of this process fox-ming 

 an irregular zone between the firm, as yet unaltered, basophile cortical 

 region and the acidophile internal region of the nucleolus. When the 

 internal pressure reaches a certain point the cortical layer is ruptured, and 

 the accumulated products are discharged into the caryolymph, partly as 

 deeply basophile spherules, partly as a clear fluid. The spherular form of 

 the discharged material is determined by the fluid character of the 

 surrounding caryolymph, and when the accumulated products of nucleolar 

 activity happen to lie within a vacuole in the nucleolus, then they here also 

 possess a spherical form. There is no indication that the acidophile ground 

 substance of the nucleolus is itself discharged. 



During the growth of the oocyte the amount of the deeply basophile 

 nucleolar substance varies greatly, but the growth of the nucleolus neverthe- 

 less shows a strict correlation with that of the egg, and is due to the steady 

 accumulation of the acidophile ground substance. 



2 f 2 



