33^ A. RICHARDS AND JAMES T. THOMPSON. 



son to the great numbers of cells in the surrounding tissues. Al- 

 though the entire mass of cells is continuously in motion, only the 

 movement of the sex-cells is at all noticeable. They are shifted 

 about by the active surrounding tissues and naturally assume 

 slightly irregular outlines at times, due to the unequal tension 

 upon the cell membrane. Through a misinterpretation of the 

 conditions within the embryo these sex-cells may easily be ac- 

 credited with peculiar powers of locomotion. A sex-cell, as a 

 slowly drifting cloud, can be seen gradually to change its posi- 

 tion; but movements of the tissue cells about it, due to their loca- 

 tion in continuous layers, are so inconspicuous as to go unnoticed. 

 Because of this, the movement of the sex-cells should be consid- 

 ered merely as the passive indication of the rate and direction 

 of progress of contiguous layers. 



SUMMARY. 



1. The earliest primary sex-cells found in Finidnliis were 

 located in the peripheral endoderm, lateral to the posterior half 

 of the 46-hour embryo. No sex-cells were observed in that part 

 of the embryo which develops from the head fold. 



2. The germinal path leads from the peripheral endoderm, into 

 the border of the undifferentiated endodermal cell mass. When 

 this cell mass splits to form gut endoderm and lateral mesoderm, 

 the sex-cells proceed medially with either layer. By the time the 

 gut is formed, these cells are lateral to it ; they all eventually be- 

 come located in the splanchnic mesoderm of this region. From 

 here the sex-cells migrate dorsal to the hind gut, thence to the 

 region ventral to the Wolffian ducts. Here they become sur- 

 rounded by peritoneal cells which form the somatic portion of 

 the gonads. From this position the germ gland anlagen are 

 shifted back to their final location dorsal to the gut.* 



3. There is very little multiplication of the sex-cells during the 

 period of migration. Division apparently takes place in the 

 extra-embryonic area, and is not renewed to any marked extent 

 until after the sex-cells become located in the germ glands. 



4. The constant distinguishing characteristics insure positive 

 identification of these cells throughout all phases of their migra- 

 tion, and leave no reason to question their identity as being the 

 "primordial germ cells" of previous writers. 



