94 THE CHANGES FROM STAGE A TO F. 



They are not therefore seen until the sixteenth somite is 

 formed. When they first appear they ai'e crowded together in 

 a mass in the endoderm at the hind end of the body; but they 

 soon begin to acquire a relation to the cells of the splanchnic 

 walls of the posterior somites. Some of them pass to the 

 surface of the endoderm and project into the somites, pushing 

 the mesoderm cells before them (Plate VIII, figs. 26 and 27, 

 gen.). I think there can be no doubt that they give rise to the 

 nuclei of the sexual cells of the adult, and I propose to call 

 them the germinal nuclei. With regard to their disposition, 

 I may give the following details : — In an embryo with eighteen 

 somites they were present in the region of the last three, viz. 

 of the sixteenth, seventeenth^ aud eighteenth. They were 

 placed in the endoderm near the layer of splanchnic mesoderm, 

 but they did not project, except in one or two cases, into the 

 cavity of the somite. 



In an embryo with twenty-one somites these nuclei were 

 present in the region of the sixteenth to the twentieth somite 

 inclusive. They were found in groups in the dorsal endoderm, 

 aud a considerable number of them projected into the somites, 

 or, in other words, had migrated from the endoderm into the 

 splanchnic mesoderm (Plate VII, fig. 26). 



The same features were presented by an older embryo of 

 Stage E, with the full number of somites. Plate VII, fig. 27, 

 represents a section through the seventeenth somite of such an 

 embryo, and shows very clearly the relations which these nuclei 

 acquire to the splanchnic mesoderm. 



The cells of the latter form capsules surrounding the ger- 

 minal nuclei, which possess but a very delicate (with difiiculty 

 visible) protoplasmic investment of their own. By the close of 

 Stage E these germinal nuclei are present in the region of the 

 sixteenth to the twentieth somite inclusive, lying partly in 

 groups in the dorsal endoderm and partly in the splanchnic 

 mesoderm. 



They are of the same size as the larger of the ordinary 

 endodermal nuclei — "014 mm. in diameter. They differ, how- 

 ever, from the latter in their evenly-rounded form, the outline 



