THE PADOGENETIC 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 
