MIGRATION OF SEX-CELLS OF FUNDULUS HETEROCLITUS. 335 



earlier origin than it has beeli possible thus far to trace them. 

 It seems not unreasonable to believe that the fore-runners of 

 these cells have been segregated at a time very early in the de- 

 velopment of the germ ring. 



DISCUSSION AND CONCLUSIONS. 



This paper attempts to identify definitely the sex-cells which 

 are present in the 24-day embryo as the " primordial germ cells " 

 of previous writers, or as the " primary genital cells " of Firket. 

 It also presents evidence on the manner in which these cells reach 

 their final destination. 



The method of embryo formation in the teleosts has a bearing 

 upon the question of sex-cell migration in Fnndulus. It will be 

 recalled that the anterior portion of the embryo is formed from 

 the head fold, which may perhaps be nothing more than a thick- 

 ening on the germ ring ; while the body or posterior portion is to 

 be regarded as the result of the developmental process termed 

 concrescence. It is only this latter portion of the body that is 

 involved in the formation of the sex-cells. The eggs have a 

 large amount of yolk, and a very distinct germ ring. As cell 

 proliferation takes place, the germ ring moves gradually down- 

 ward over the yoke mass. The primitive streak moves back- 

 ward and receives the converging limbs of the germ ring 

 posteriorly. The material of the halves of the germ ring, after 

 fusion, is differentiated into the embryo posterior to the head 

 process. The rudiments of the embryo body are not clearly 

 marked out in Fnndulus until the germ ring is completely closed. 



The earliest primary sex-cells which we have located are from 

 embryos in which the germ ring has been closed but a few hours, 

 and in which the tail is just beginning to elongate. Their posi- 

 tion in the extra-embryonic region lateral to the undifferen- 

 tiated endodermal cell mass at the posterior half of the embryo 

 is indicated in Fig. 20. In other embryos of the same stage of 

 development, numerous primary sex-cells are present in prac- 

 tically the identical relation to the embryo that is clearly demon- 

 strated in Fig. 20. These sex-cells invariably lie just above the 

 periblast and are associated with the sheet of cells which is a 

 lateral expansion of the undifferentiated endodermal cell mass 



