UROGENITAL ORGANS. \2J 



The former retains its connection with the menesophros ; but 

 the Miillerian duct loses all connection with the Wolffian body, 

 and opens into the coelom by means of the fused nephrostomes 

 of the pronephros, the ostium tubae abdominale. 



In all other vertebrates a Miillerian duct is formed, ac- 

 cording to the older accounts in the same way as in the elas- 

 mobranchs, but according to most recent writers as a new 

 formation. An ingrowth of ccelomic epithelium begins at the 

 anterior end of the mesonephros, and continues backward until 

 the cloaca is reached. During its growth it becomes tubular, 

 and at its anterior end it opens widely into the coelom, behind 

 into the cloaca. It thus forms a Miillerian duct essentially like 

 that of elasmobranchs in structure, but greatly different in 

 origin. In all cases the duct receives, in addition to the epithe- 

 lium, mesenchymatous tissue, which makes up the bulk of its 

 walls. 



The Miillerian duct in the female of both elasmobranchs. 

 and of higher vertebrates henceforth functions as an oviduct. 

 The eggs, which escape from the ovary, pass into its funnel 

 (ostium), and are thence conducted to the exterior. It may 

 remain a simple tube, or more usually distinct regions may be 

 specialized in it, each with distinct functions. Most constant 

 of these, except in mammals, 1 are regions which secrete the 

 protective envelopes shell, egg membrane, etc. around the 

 egg. In those forms which bring forth living young (mammals, 

 many elasmobranchs, etc.) one portion of the duct becomes 

 specialized to retain the egg during its development, and re- 

 ceives the name uterus in those forms where the growing young 

 acquires attachment to the lining walls. With the develop- 

 ment of the uterus that portion of the Miillerian duct in front 

 is called the Fallopian tube, while the post-uterine portion forms 

 a vagina. In the lower mammals as in the lower vertebrates 

 the Miillerian ducts usually remain distinct from each other 

 throughout their length, the result being two uteri and two va- 

 ginae ; in the higher mammals a fusion of the posterior end of 

 the ducts of the two sides occurs, resulting in a single vagina, 

 and usually of an unpaired uterus, the latter showing more or 



1 The monotremes differ from the other mammals in this respect. 



