484 ROBERT W. HEGNER 



at the posterior end of the egg and the embryo has just started 

 to form; no further increase in number occurs until the larval 

 stage is reached and the definitive germ glands are established. 

 As soon, however, as the embryo has reached a certain develop- 

 mental stage, the germ cells migrate into it, and it looks very 

 much as though they remain quiescent until the somatic cells 

 are "able to protect, nourish, and transport" them. 



The number of primordial germ cells during the 'period of 

 rest' is perhaps most definitely known in Miastor, where, as one 

 group of eight and later as two groups of four each, they are 

 present throughout a large part of embryonic development. 



In vertebrates also, a long period exists, during which division 

 of the primordial germ cells does not take place (fig. 24) and, at 

 least in several species, certain cell contents (the mitochondria) 

 remain in an indifferent condition (Rubaschkin, '10; Tschaschkin 

 '10; fig. 23, B). These facts all indicate that these cells remain 

 in a primitive condition and do not undergo the histological 

 differentiations characteristic of somatic cells, a view which, 

 however, has been objected to (Eigemann, '96). The disap- 

 pearance of the Keimbahn-determinants and the yolk globules 

 of vertebrates during this period have suggested that these sub- 

 stances are nutritive in function, furnishing energy to the mi- 

 grating germ cells. 



The fact of this long rest period, followed by rapid mul- 

 tiplication of the oogonia and spermatogonia, during which no 

 important specializations occur, and later succeeded by the 

 remarkable changes which occur in both the oocytes and sper- 

 matocytes has led to the suggestion (Montgomery, '11, pp. 790- 

 792) that in the germ cell cycle there is a series of changes parallel 

 with that of the somatic cycle. In the development of both 

 cycles preformation and epigenesis proceed at the same time. 

 The chromosomes seem to be preformed elements of the germ 

 cells, since they are apparently the most stable constituents. 

 The cytoplasm, on the other hand, undergoes a series of epigenetic 

 changes such as the formation of an idiozome, the development 

 of mitochondria, the appearance of a sphere and the metamor- 

 phosis of the spermatozoon. 



