II 



MEIOSIS IN THE FEMALE 



63 



nature (amphinucleoli), consisting of a plastin groundwork (plasmosome), 

 covered or impregnated with chromatin or a chromatin-Uke substance ; 

 or the two constituents may be separate, so that the nucleolus consists 

 of two parts, a chromatin and a plastin portion. These remarks refer 

 especially to the main nucleolus, which persists right through the growth 

 period. In many animals the secondary nucleoli which develop later 



^K. 



1 I 





Fig. 26. 



Showing the fate of the nucleolus in the oogenesis of Daphnia pulex. (After Kuhn, A.Z., 1908.) A, the 

 large central nucleolus is beginning to break up into much smaller bodies which are spreading over the thin 

 threads representing the chromosomes ; B, C, the continuation of this process. In C the nucleolus has completely 

 disintegrated into granules or droplets which conceal the chromosomes. D, the condensed chromosomes 

 (c) embedded in the disintegrated nucleolar mass ; E, the meiotic division. Note the mass of nucleolar 

 granules left in the cytoplasm by the rupture of the nuclear membrane. A is drawn under a higher magnification 

 than the remaining figures, which are all to the same scale. 



appear to be purely of the nature of plasmosomes — e.g., Cyclops brevicornis 

 (Hacker, 1893). 



(3) The Connection between the Germinal Vesicle and Yolk Formation 



The co-existence in the primary oocyte of two unique cytological 

 occurrences, namely, the germinal vesicle and the enormous growth of 

 the cell with its formation of reserve food material, naturally suggests a 

 causal connection between them. We have already drawn attention to 

 the fact that the diffusion of the chromatin in the ordinary resting 

 nucleus has the result of increasing its area in proportion to its mass, 

 and thus of favouring active metabolism. The excessive diffusion of 



