382 HISTORY OF THE HUMAN BODY 



do not develop any such method. Through this close con- 

 nection between the originally distinct reproductive and uri- 

 nary systems it results that both the anterior part of the meso- 

 nephros and the mesonephrotic duct become, apart from their 

 urinary function, accessory reproductive organs, the former 

 serving as a " sexual kidney," and the latter as a ductus 

 deferens, or excurrent seminal duct. 



In the female selachian a different modification takes place, 

 seemingly not due to association with the urinary system, but 

 proven to be so by the developmental history of the parts. 

 The ovary of the adult occupies about the same position as do 

 the testes of the male, but shows no direct connection with the 

 anterior part of the kidney. In place of this there appears on 

 each side a long tube running along the side of the mesone- 

 phrotic duct and opening posteriorly into the cloaca beside 

 that of its associate. This is the oviduct, or " Midler's duct" 

 of many writers. At the free anterior end, which extends to 

 almost the forward limits of the ccelom, it opens by an ex- 

 panded mouth, ostium tubce, directly into this latter cavity 

 and receives into this the mature ova which become released 

 from the ovary and wander about in the cozlom in the primitive 

 fashion. The oviduct arises in the embryo as a tube seg- 

 mented off longitudinally from the mesonephrotic duct by the 

 common method of the development of two longitudinal folds 

 opposite one another, and thus points to a period at which 

 the ova as well as the spermatozoa were conveyed to the cloaca 

 through the mesonephrotic duct. The ostium is probably an 

 enlarged and specialized nephrostome, associated with a single 

 nephridium,* and it is thus easily imagined that the primary 

 conditions in the female corresponded closely to that of the 

 male, but, that owing to the greater size of the products to be 



* The not infrequent occurrence, even in the human subject, of two 

 ostia upon one side may possibly be the result of the retention of two 

 nephrostomes instead of a single one, or it may be simply an anomaly 

 like the multiplication of digits on other parts. If it be the first it con- 

 cerns a very ancient bit of history, and suggests an extreme degree of 

 reversion. 



