THE GENERATIVE ORGANS. 95 



being perfectly smootli, 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 (Plate VIII, 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 (Plate IX, 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 ) 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 (Plate IX, 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 attached to it 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. 



The Somites of the Twenty-first Pair, or somites of the anal 

 papillae, never become divided into two parts. The median divi- 

 sion remains in connection with the lateral (Plate IX, fig. 44), 

 which, however, as in the case of the other somites, acquires 

 a ventral diverticulum. This hugs the outer side of the nerve- 

 cord, and acquires in late embryos of Stage f an external 

 opening, which, however, is much nearer the middle line than 



