60 GERM-CELL CYCLE IN ANIMALS 
egg, and until the posterior or tail end has been 
pushed around for a considerable distance on the 
dorsal surface. A broadening and a_ shortening 
of this germ-band then takes place so that the pos- 
terior end of the embryo coincides with the posterior 
end of the egg and the edges of the embryo grow 
laterally around the egg until they meet in the 
median dorsal line. Meanwhile various changes 
have taken place within the embryo, among which 
is the formation of the germ glands or ovaries. 
Returning now to a consideration of the germ cells, 
we shall see that it is possible to trace the descendants 
of the primordial germ cell with comparative ease. 
This cell divides by mitosis, forming two odgonia 
approximately equal in size (Fig. 18). These two 
then produce four odgonia of the second order 
(Fig. 19), and these in turn increase by mitosis, 
forming eight odgonia of the third order (Fig. 20). 
When this stage is reached a period sets in during 
which the odgonia do not divide, but are apparently 
passively carried about by the somatic tissues as 
shown in Fig. 21, where they occupy a position 
near the end of the tail fold. 
One of the most satisfactory conditions in the 
keimbahn of Miastor is the comparatively large 
size and peculiar structure of the primordial germ 
cells leaving in the mind of the observer no doubt 
as to the identity of the cells concerned. Through- 
out the entire embryonic development of this insect 
the germ cells are considerably larger than any of 
the somatic cells. The nuclei are correspondingly 
