66 RICHARD EVANS. 



The Development of tlie Genital Ducts.— In one of 

 two embryos slightly older than those of which sections are 

 shown in figs. 13 and 14, there were twenty-six pairs of 

 somites actually formed, and in the other twenty-seven. At 

 the posterior end, the former had two pairs of somites 

 possessing no germinal nuclei, while the latter had three 

 pairs devoid of them. Of these three pairs of somites which 

 develop no germinal nuclei, the anterior one {som"'^) gives 

 rise to the genital ducts, the middle one {som."'''-) produces 

 the last renal organ, while the hindermost disappears in the 

 female, and gives rise to the accessory glands in the male. 

 It is not quite correct to say that the entire somite becomes 

 the genital duct, for the ventral portion becomes obliterated 

 by the coming together of its walls, of which the cells form 

 at a later period the lining of the blood-spaces which develop 

 in that region. An appendicular outgrowth, which is never 

 separated from the median coelom, forms in the genital somite, 

 as in any obher; but instead of opening to the exterior at 

 the base of the legs, the two tubes debouch together in 

 the median line. The inner ends of the two ducts thus 

 formed come together in the female and unite, subsequently 

 opening by a common pore into the cavity of the ovary. In 

 the male they do not communicate with each other at any 

 time, but each duct acquires a separate opening into the 

 testis of its own side. 



Further Modifications of the Oviducts. — As regards 

 the external ends of the oviducts, the stage already described 

 almost corresponds to the structure in the adult. The ecto- 

 dermal ingrowth, such as it is, forms the extremely short 

 vagina of the adult, and no more. In one of my embryos 

 there is a distinct line of demarcation between the ectoder- 

 mal and mesodermal constituents. The vagina lies almost 

 horizontally, and the ectodermal portion of its dorsal wall, 

 derived from the posterior lip of the external opening, is 

 decidedly shorter than that of the ventral wall, Avhich pushes 

 its way forward as a tongue-shaped structure, situated in the 

 median line. There seems to be no doubt that von Kennel 



