GERM-CELL HISTORY IN THE BROOK LAMPREY 13 



of the table, in which case factors influencing the animal during 

 development may be responsible for the resulting sex. The 

 possibility is not excluded of sex being the result of the joint 

 action of hereditary and external factors. 



Whether the germ cells of the larval lamprey eventually give 

 rise to ova or to spermatozoa, their early history appears to be 

 the same. Sooner or later some of the primordial germ cells 

 transform into oocytes in practically all the larvae, irrespective 

 of whether the larvae which bear them eventually become ma'es 

 or females. In the present work the history of the germ cells 

 has been studied up to a period when males and females can be 

 distinguished by an examination of the germ glands. 



The scheme in table 2 presents in a graphic form the history of 

 the germ cells in the lamprey in both males and females in rela- 

 tion to the development of the body. As here shown, the life 

 of the animal extends over a period of five years, and only a 

 small part of the life-cycle is spent in the adult stage. The 

 scheme also forms a basis for the terminology employed in the 

 subsequent pages. 



B. Origin and early history of the germ cells up to the beginning of 



sex differentiation 



1. Observations on Entosphenus wilderi. During cleavage and 

 gastrulation in the lamprey all cells are more or less laden with 

 yolk and, if the germ cells are segregated at this time, no char- 

 acter has been discovered by means of which they may be dis- 

 tinguished from other cells. But after the mesoderm begins to 

 separate from the entoderm certain large cells occur which may 

 be identified as the primordial germ cells. 



a. Embryo 191 hours old (fig. 5). A camera-lucida drawing 

 of an embryo 191 hours old (two or three days before hatching) 

 s shown in figure 5, and figure 11 shows a section along the line 

 ab of figure 5. In this embryo the mesoderm has already sepa- 

 rated from the entoderm cranially, but at the caudal end, i.e., 

 in the region from which the section (fig. 11) was taken, there 

 seems as yet to be no division line between the two layers. The 

 mesentoderm in this region extends dorsad as two ridges, one 



