386 



Dr. G. C. Chubb. 



[Dec. 20, 



On the completion of yolk formation all indication of nucleolar activity 

 abruptly ceases. The supply of the basophile constituent of the nucleolus 

 also ceases, but the acidophile constituent continues to be deposited on to the 

 nucleolar surface, where, no longer being incorporated by the now inactive 

 nucleolus, it forms lens-shaped accumulations. 



The selective action shown by the nucleolar material, together with the 

 strict correlation the growth of this structure shows to that of the egg, 

 excludes the possibility of this substance being derived directly from a source 

 outside the cell. The fact that the ground substance continues to be deposited 

 on to the nucleolus after the completion of all cytoplasmic activity, together 

 with the close relation this structure shows to the chromatin in earlier stages, 

 points to the latter as its source. The failure of the supply of the basophile 

 constituent on the completion of yolk formation indicates its origin from 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. 



The Chromatin. — With the increase in metabolism which accompanies the 

 growth of the oocyte there is a corresponding finer subdivision, and more 

 equal distribution of the chromatin. This endeavour to increase the facilities 

 for reaction with the surrounding caryolymph indicates that the chromatin 

 obtains its food material by active incorporation instead of passively by 

 deposition. The absence of any reciprocal action on the part of the food 

 material of the chromatin indicates for this substance a source outside the 

 cell. The chromatin is, therefore, the cell-structure with which the raw 

 food material supplied by the parent organism first comes into relation. 



During the gradual accumulation of metaplastic material in the cytoplasm 

 which marks the period preceding yolk formation, the chromatin is sparsely 

 scattered and relatively faintly stained. With commencing yolk formation 

 the staining capacity, and hence also the quantity visible, of the chromatin 

 rapidly increases, and at the same time basophile droplets appear on the 

 chromatin threads and increase in number and size during yolk formation. 



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, and 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 



