34-8 A. RICHARDS AND JAMES T. THOMPSON. 



PLATE IV. 



FIG. 20. A diagrammatic reproduction of embryo B' 21 (46 hours) dem- 

 onstrating the distribution of the sex-cells at this early stage. The variation 

 in anterior-posterior development is very noticeable and is explained fully in 

 the text. X 65. 



FIG. 21. Reproduction of embryo B' 26 (105 hours) constructed simi- 

 larly to Fig. 20. Showing the positions of the sex-cells as being more medially 

 placed than in the earlier stage. The cells on the right are not as far along 

 with migration as those on the left ; the former group migrated with the 

 mesoderm and the latter followed the gut endoderm. X 65. 



