MIGRATION OF SEX-CELLS OF FUNDULUS HETEROCLITUS. 



tion of the coelome, it is a noteworthy fact that they never occur 

 within the somatic layer after differentiation. 



From this position lateral to the hind gut the cells are in the 

 general dorsal movement of the mesoderm which eventually re- 

 sults in the formation of the intestinal mesentery. The cells 

 from either half of the embryo remain apart and seem to lie in 

 separate streams of mesodermal cells which are flowing toward 

 the Wolffian ducts. But although there may be a pause here, at 

 no time do the sex-cells appear to establish any intimate relation 

 with the cells of these ducts (Text Fig. D). From the evidence 

 at hand, an explanation of the function of these cells which 

 makes them contributory to the development of the already well- 

 formed Wolffian ducts, as suggested by certain investigators, does 

 not seem plausible in Fnndnlns. 



As the sex-cells reach a position nearly ventral to the Wolffian 

 ducts they become surrounded by a single layer of peritoneal 

 cells. This covering develops until the position of the future sex 

 organs is attained ; the sex-cells then rest in sac-like protuber- 

 ances from the peritoneum, the germ gland anlagen. Assisting 

 in the movement which brings the sex-cells into their future posi- 

 tions, are several factors entirely external to the germ glands. 

 For example, there is a rapid proliferation of the loose mesen- 

 chyme dorsal to the gut and the development of the swim blad- 

 der which results in a median down pushing. The ventral 

 movement (Text Figs. D and ) from the region of the Wolf- 

 fian ducts is clearly due to the wedge-like effect produced by the 

 growing swim bladder. That this process is necessarily passive 

 is evident from the fact that amoeboid activity of the cells in- 

 cluded within the germ gland would be unable to produce any 

 change in its position. 



From the evidence in Fundulus it is apparent that the sex-cells 

 enter the embryo and are located in the germ glands by the same 

 forces that are influential in the distribution of the other organ 

 forming substances of the body. Their " migration " is not to be 

 looked upon as different from that of any other group of cells. 

 But while the sex-cells are not amoeboid, there is nevertheless 

 reason for misunderstandings which have arisen regarding their 

 activities. In the first place they are relatively few in compari- 



