590 Gideon S. Dodds. 



embryo, and throughout the series of stages, they may be traced 

 as following a perfectly definite path. 



The next point is to determine whether these are really germ- 

 cells. Are they actuall}^ the cells which later give rise to oogonia 

 and spermatogonia? Though I have not traced them through 

 the later stages, they correspond so well with cells which in tele- 

 osts, as well as in other groups of vertebrates, have been defi- 

 nitely traced to these stages, that there can be little doubt of 

 their identity. It would not have seemed necessary to raise this 

 question at all, were it not for the contention of some recent 

 writers, that these so-called 'sex-cells,' of early stages, including 

 the 'Ureier' of Waldeyer, are not germ-cells at all, but simply 

 hypertrophied cells which later entirely degenerate. 



e. Period of rest of the germ-cells. A period of rest from division 

 of considerable duration seems to be general during early stages 

 of the germ-cells of vertebrates. In some invertebrates, at least, 

 the same is the case. A phenomenon of such general occurrence 

 should be of some significance. 



In all vertebrates examined this period corresponds to the time 

 during which the germ-cells are in active migration, and it has 

 been suggested that possibly the energy of the cells is expended 

 in locomotion rather than in growth and cell division. 



It has also been pointed out, that this period corresponds to 

 the period of embryo formation. This relation is certainly an evi- 

 dent one in many forms, and may be of some significance. In 

 Lophius it is very apparent. At stage 6, which probably marks 

 the beginning of the period of rest, the formation of the embryo 

 has scarcely begun. By stage 14, which probably about ends the 

 period of rest, the organs and tissues of the embryo are quite 

 definitely formed. Thus it appears, as suggested by others, 

 that the germ-cells are early set apart, after which they rest while 

 the embryo is being built for their reception. Such observations 

 give support to the view that germ-cells are really primitive, 

 generalized cells, early set apart so that they may retain the pos- 

 sibilities of development which become lost during histological 

 differentiation of somatic cells. This may be the true reason for 



