346 A. RICHARDS AND JAMES T. THOMPSON. 



PLATE III. 



FIG. 13. Sex-cell from the so-hour embryo B' 23. It lies in the extreme 

 edge of the lateral mesoderm. just dorsal to its separation from the periblast. 

 This cell lies in that portion of the mesoderm which will give rise to the 

 splanchnic layer. X 700. 



FIG. 14. A sex-cell in the splanchnic mesoderm of the io5-hour embryo 

 B' 26. The cell is just medial to the point of differentiation between the 

 somatic and splanchnic layers. This stage also shows an advance over the 

 one illustrated in Fig. 13, in that the gut and Wolman duct have taken form. 

 X 700. 



FIG. 15. A group of sex-cells from the region anterior to that drawn in 

 Fig. 14. Here the sex-cells are approaching the side of the hind gut. X 700. 



FIG. 1 6. A sex-cell from embryo B' 26, in the region anterior to those 

 illustrated in Figs. 14 and 15. The more medially placed cell is in the 

 splanchnic mesoderm lateral to the gut. As this layer grows up over the gut 

 to form the dorsal mesentery, the sex-cells will naturally be brought to lie in 

 this region. X 700. 



FIG. 17. Four sex-cells from the 6-day embryo B 30 in the mesentery 

 dorsal to the gut. This shows in detail the region included in the rectangle 

 in Text Fig. C. The extraordinary width of the mesentery at this stage is 

 doubtless due in part to the presence of the sex-cells. X 420. 



FIG. 1 8. This figure illustrates the extremes of the migration at this stage. 

 No cell was found at any earlier stage than the one near the gut, none later 

 than that ventral to the Wolffian duct. From embryo B 30. X 420. 



FIG. 19. Showing the development of the gonads in the 1 3-day embryo 

 B 42. The sex-cells are fixed in sac-like protuberances from the peritoneum. 

 From this figure it is possible to obtain an idea of the effect produced by the 

 rapid growth of .the swim bladder, in literally pushing the gut and all related 

 tissues ventrally. It is also interesting to observe that the peritoneal cover- 

 ing renders this cell dependent upon surrounding tissues for movement to 

 its final position dorsal to the gut. X 420. 



