342 A. RICHARDS AND JAMES T. THOMPSON. 



DESCRIPTION OF ILLUSTRATIONS. 



PLATE I. 



FIG. i. A typical primary sex-cell from a los-hour embryo (B' 26). 

 Showing the large centrosome, two nuck-oli and chromatin knots scattered 

 oVer the periphery of the nucleus. X 1420. 



FIG. 2. A typical sex-cell from a 6-day embryo (B 30). Chromatin gran- 

 ules finer than in the preceding cell. X 1420. 



FIG. 3. Sex-cell from a loo-hour embryo (B' 25-3). From the extra- 

 embryonic region. It is closely associated with the periblast. In other sec- 

 tions the peripheral endoderm may be seen out over this area. X 1420. 



FIG. 3. Sex-cell from a g-day embryo (B 34). Chromatin arrangement 

 is intermediate between that found in Figs, i and 2. X 1420. 



FIG. 40. Surface view of a nucleus from a sex-cell in a io5-hour embryo, 

 showing the chromatin knots in surface view. X 1420. 



FIG. 4&. From the same nucleus as the one used in Fig. 40. Obtained by 

 focusing upon the center of the nucleus, illustrating the chromatin knots in 

 optical section. X 1420. 



FIG. 5. Showing the sex-cells in the germ gland anlagen dorsal to the 

 hind gut of a 24-day embryo. The function of the peritoneal cells at this 

 stage is quite evident in this illustration. The sex-cells are completely sur- 

 rounded by mesoderm. X 700. 



FIG. 6. Transection through a 24-day embryo (B 65-2) taken at the posi- 

 tion indicated by the plane X-X' in Fig. 7. Showing the position of the 

 gonads in relation to other parts of the embryo ; especially as regards the 

 developing swim bladder. X 120. 



FIG. 7. Longitudinal section lateral to the median line of the 24-day em- 

 bryo (B 65). Illustrating the position of the gonads as being the same as in 

 the adult. Migration ceases at this point. The activities within the gonads 

 after they have reached this point of development will not be considered at 

 this time. X 25. 



