ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 555 



this diffusion gives rise is the yolk-nucleus. As the cytoplasm becomes 

 still more fluid, the yolk-nucleus becomes a concavo-convex lens em- 

 bracing the spherical germinal vesicle. The yolk-nucleus is simply a 

 region of the cytoplasm on to which waste material discharged from the 

 nucleolus has diffused, and its subsequent movement to the periphery 

 and its other changes are all determined by the progressive change in 

 the physical consistence of the cytoplasm which precedes and accom- 

 panies yolk-formation. 



The spherical form of the nucleolus, a form in which minimum 

 surface area is associated with maximum 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. 



The 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 consistence. Nucleolar activity, which proceeds 

 from within outwards, results in the breakdown of the basophile con- 

 stituent, the products of which are eventually discharged into the 

 caryolymph. On the completion of yolk-formation all indication of 

 nucleolar activity abruptly ceases. The supply of the basophile con- 

 stituent of the nucleolus also ceases, but the acidophile constituent 

 continues to be deposited on the nucleolar surface, where, no longer 

 being incorporated by the now inactive nucleolus, it forms lens-shaped 

 accumulations. The acidophile constituent is probably due to the 

 chromatin, the basophile to the cytoplasm. The steady growth of the 

 nucleolus is not due to the accumulation of waste material, but to an 

 increased production of the " ground substance " by the chromatin in 

 order to cope with the ever-increasing production of waste material by 

 the cytoplasm of the growing egg. From the nucleolus this waste 

 material, now presumably inert and harmless, is discharged into the 

 cytoplasm, where it slowly dissolves away. 



As metabolism increases, the chromatin is more sub-divided ; this 

 facilitates reaction with the surrounding caryolymph, and indicates that 

 the chromatin obtains its food -material by active incorporation. The 

 source is outside the cell, and the chromatin is the cell-structure with 

 which the raw food-material supplied by the parent organism first comes 

 into relation. The elaborated food-material is passed on from the 

 chromatin, in part to the nucleolus, but mainly to the cytoplasm. 

 During the period preceding yolk-formation, which is marked by the 

 progressive accumulation of metaplastic material in preparation for this 

 process, the avidity with which the cytoplasm takes up the products of 

 chromatin activity causes the latter structure to appear faintly stained. 

 With the commencement of yolk-formation this avidity is greatly 

 reduced, and the products of chromatin activity, now being produced in 

 excess of the requirements of the cytoplasm, accumulate on the chromatin 

 threads, causing the increase in the staining capacity of the latter and 

 the formation of the basophile droplets. 



Throughout the whole growth-period of the oocyte there is a gradual 



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