THE PiEDOGENETIC FLY, MIASTOR 63 



During the shortening and broadening of the germ 

 band the group of eight oogonia of the third order 

 becomes separated into two rows of four each — one 

 row on either side of the body in the region of the 

 eleventh segment (Fig. 22). Each group of four 

 oogonia then becomes surrounded by a layer of 

 mesoderm cells and forms a more or less spherical 

 body which may now be called an ovary (Fig. 23). 

 Soon after this occurs, the oogonia begin to divide 

 again (Fig. 23, a) and by successive mitoses there 

 are formed oogonia of the fourth, fifth (Fig. 24), and 

 sixth orders. This completes the number of oogonia, 

 which is typically thirty-two in each ovary, and 

 provides us with the only case thus far on record 

 where the number of oogonial divisions during the 

 multiplication period in the history of the germ 

 cells is known (Fig. 26). 



There are then six of these oogonial divisions 

 between the formation of the single primordial germ 

 cell and the production of the complete number of 

 oogonia in the two ovaries. Some of the oogonia of 

 the fifth order may be prevented from dividing, in 

 which case of course there are less than thirty-two 

 germ cells in each ovary. And not all of the oogonia 

 in the ovary succeed in developing into oocytes and 

 larvae, since a struggle for supremacy takes place 

 among the germ cells resulting in the survival of only 

 a few offspring, as may be determined by the fact, 

 already referred to, that one larva gives rise as a 

 rule to only from five to seventeen daughter larvae. 

 Each oogonium that succeeds in developing becomes 



