612 ADAM SEDGWICK. 



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 

 being perfectly smooth, and in the fact that they lie together in 

 groups of three or more. 



With regard to their origin, they come, as do all the tissues 

 of the hind end of the body, from the primitive streak ; and 

 though there are in the primitive streak in Stage c large round 

 nuclei of their aspect, it is not until Stage d that they can be 

 distinguished in the endoderm with the above-mentioned 

 characteristics. 



The median divisions of the somites in the generative region 

 remain continuous with the lateral (nephridial) portions until 

 the close of Stage e. Early in Stage f they have completely 

 separated from the latter (PL XXXVI, fig. 41), and now rapidly 

 become reduced in size, so that by the close of Stage f they have 

 the form of somewhat triangular structures placed in the ventral 

 wall of the heart and pericardium (PI. XXXVII, fig. 43). 

 Inasmuch as the gut has now completely separated from their 

 ventral walls, the generative nuclei have entirely lost their con- 

 nection with the endoderm — their place of origin. They still 

 lie in the ventral walls of the somites, — a position which they 

 maintain throughout life. 



In the next Stage (g, to be more fully described in Part IV 

 of this series) the successive dorsal (generative) portions of 

 the somites of the same side unite with one another, in con- 

 sequence of the breaking down of the intervening segmental 

 septa, so as to form two tubes (PI. XXXVII, figs. 47, 48), 

 which are the generative glands. The generative glands 

 separate from the floor of the pericardium, except at their 

 front end, where they remain connected throughout life. They 

 thus have the form of two tubes closely applied together and 

 placed in the dorsal region of the central compartment of the 

 body cavity. 



