THE P^DOGENETIC FLY, MIASTOR 59 



later stages, is the primordial germ cell. At this 

 time, then, the egg consists of one primordial germ 

 cell provided with a nucleus with an undiminished 

 amount of chromatin, and a syncytium containing 

 seven nuclei of which the sister nucleus of the primor- 

 dial germ cell contains a complete supply of chroma- 

 tin, whereas the other six nuclei have lost part of 

 this chromatin material. Reference to the diagram 

 on page 65 will assist in making more clear this stage 

 and the stages yet to be described. 



The next developmental process is the mitotic 

 division of the seven nuclei in the syncytium thus 

 producing a fifteen-cell stage (Fig. 17). The sister 

 nucleus of that of the primordial germ cell now under- 

 goes a chromatin-diminution process and the other 

 six nuclei in the syncytium pass through a second 

 chromatin-diminution process. As a result every 

 nucleus in the egg has lost a part of its chromatin 

 except that of the primordial germ cell which still 

 contains a complete amount. The further history 

 of the somatic nuclei does not differ essentially 

 from that of the somatic nuclei in other insects. 

 They increase in number by mitosis, migrate to 

 the periphery, and there are cut off by cell walls 

 forming a single layer of cells over the entire surface 

 except where interrupted at the posterior end by 

 the primordial germ cells. Next, a thickening of 

 the cells occurs on the ventral surface, thus forming 

 the ventral plate. From this plate most of the 

 embryo arises ; it lengthens until the anterior or 

 cephalic end almost reaches the anterior end of the 



