98 bulletin: museum of compaeative zoology. 



tained within the nucleolus, which will be considered later. During the 

 growth of the nuclear reticulum, then, there appears to be a concentra- 

 tion of nucleic acid within the nucleolus, while the reticulum itself 

 and all the other granules within the vesicle suffer a decrease in their 

 percentage of phosphorus. 



The condition of the germinative vesicle shown in Figure 48 

 (Plate 6) is found in ova about 150 fx in diameter, in which the yolk 

 bodies are just beginning to be formed. During all of the subsequent 

 growth of the ovum, while it doubles its diameter, the germinative vesicle 

 decreases in size, usually diminishing to a little less than half its former 

 diameter (see Table, p. 100). During this shrinking, the vesicle exhibits 

 the wavy and stellate outline characteristic of the stages preceding tlie 

 maturation of ova (Plate 6, Figs. 49, 50) ; here, however, the shrivel- 

 ling is associated not with maturation but with yolk formation. 



This shrinking of the germinative vesicle and the formation of the 

 maturation nucleus from it have been studied by Davidoff (pp. 156-159), 

 according to whom the process is very complex and quite unique. First, 

 the membrane disappears, and the major portion of the contents of the 

 germinative vesicle forms the " ergoplasma," or active cytoplasm, which 

 gradually becomes disseminated between the yolk bodies. The nucleo- 

 lus is the only structural element that remains behind, and it under- 

 goes the most complex modifications. It takes a deep stain, and becomes 

 shrivelled so that its outline is irregular and at times stellate. He says 

 he believes that there is an actual decrease in the volume of the nucleo- 

 lus accompanying these morphological changes, but his figures do not 

 bear him out in that statement. The next change is, that within the 

 shrivelled nucleolus small chromatic granules are differentiated, which 

 soon aggregate to form a compact chromatic body. This itself sub- 

 sequently becomes vesicular and contains a central granule. A few of 

 the chromatic granules do not take part in the formation of the compact 

 chromatic body, but initiate the formation of a dense chromatic network, 

 with which the whole nucleolus becomes filled. At the same time a 

 membrane is distinctly differentiated about the periphery of the nucleo- 

 lus and we have developed from it a nucleus which is neither germinative 

 vesicle nor nucleolus, but the maturation nucleus or " Polkern." 



In Distaplia occidentalis, most of the stages discussed by Davidoff are 

 abundantly represented; but I must differ from him in their interpreta- 

 tion and in the order of their sequence. I will first describe the process 

 as I conceive it to be, and then compare my results with those of 

 Davidoff. 



