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 oogonia 

 approximately equal in size (Fig. 18). These two 

 then produce four oogonia of the second order 

 (Fig. 19), and these in turn increase by mitosis, 

 forming eight oogonia of the third order (Fig. 20). 

 When this stage is reached a period sets in during 

 which the oogonia 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 



