332 A. RICHARDS AND JAMES T. THOMPSON. 



the lateral mesoderm fuse above the gut, the formation of the 

 dorsal mesentery would result and the position of the sex-cells 

 would be identical to that found in later stages. Ex. Figs. 17 

 and 18. 



6-Day Embryo. 2.6 Mm. Long. Text Fig. C represents the 

 position of the sex-cells as found in the 6-day embryo. At this 

 time they are apparently in a state of rapid migration from the 

 loose mesenchyme dorsal to the hind gut, to the positions ventral 

 to the Wolffian ducts. Because of the laterally compressed con- 

 dition of the embryo, which was due to the softness of the paraffin 

 at the time of cutting, the transections are not exactly typical. 

 However this embryo has been used since it represents most 

 clearly the transitional stage between those figured in Text Fig. 

 B and D. Figs. 17 and 18 are detailed drawings of the 6-day 

 stage. They illustrate the complete extent of the migration in the 

 mesentery. The majority of the sex-cells were in the dorso- 

 ventral position indicated by the cells in Fig. 17, while only a few 

 were in that shown in Fig. 18. The more anterior sex-cells were 

 farther along in the germinal path (being nearer the Wolffian 

 ducts) than the more posterior ones. The position of the germ 

 gland anlagen ventral to the Wolffian ducts is illustrated in Text 

 Fig. D. 



i^-Day Embryo. 4 Mm. Long. Text Fig. E represents the 

 position of the germ gland anlagen as found in the 1 3-day embryo 

 B 42 (4 mm.). The rectangle in Text Fig. E includes the region 

 which is drawn in detail in Fig. 19. Rarely more than one sex- 

 cell is found at this stage in any one section of a germ gland 

 anlage. The anlagen are little more than protuberances from 

 the peritoneum, containing relatively few peritoneal cells (al- 

 though the sex-cells are surrounded by them) and they have not 

 yet reached the future position of the gonads. It is obvious that 

 the sex-cells which are contained in the peritoneal sac are all 

 pushed ventrally by the developing swim bladder. One cell was 

 observed in the position indicated by the cross in Text Fig. E. It 

 was not included in the peritoneal sac and seemed apparently 

 helpless in the loose mesenchyme ventral to the Wolffian duct. 

 This cell had been delayed in reaching this position, had not been 

 included in the sac, and in consequence of this fact it had not been 



