814: COMPARATIVE ANATOMY 



ductive system. It will be noted, then, that sperm can pass throughout 

 the lumen of a tube the entire distance from the origin to its exit from 

 the body. 



The cephalic end of the Wolffian duct also becomes purely repro- 

 ductive in the male, it being considerably coiled to form the epididymis. 

 (Fig. 458.) In the amniotes, where the hinder portion of the mesone- 

 phros is supplanted by the true kidney (metanephros), the whole 

 Wolffian duct is a sperm duct (vas deferens) in the male, while in the 

 female it largely or completely degenerates. In the amphibia and elas- 

 mobranchs the hinder end of the duct is both reproductive and excretory 

 in the male. In the female it is purely excretory. 



"In the ichthyopsida, other than elasmobranchs and amphibia, the 

 sperm is carried to the exterior in other ways, and there is no connection 

 of the testes with the excretory organs. In the cyclostomes the sperm 

 escapes from the testes into the coelom and then is passed to the exterior 

 by way of the abdominal pores which in the lampreys open into a cavity 

 (sinus urogenitalis, Fig. 457) which also receives the hinder ends of the 

 Wolffian ducts. In the myxinoids the pores are united, and they open to 

 the exterior behind the anus and between it and the urinary openings." 



OVIDUCTS 



As already stated, in many forms the Miillerian duct is the direct 

 result of the splitting in two of the pronephric duct which then serves as 

 the oviduct. At its separation from the Wolffian duct the Miillerian 

 duct opens into the coelom by means of the pronephric tubules and their 

 nephrostomes. These then flow together and form a larger opening 

 called the ostium tubae abdominale, on each side. (Fig. 458.) In the 

 elasmobranchs the ostia are usually united ventral to the liver. The eggs 

 which are thrown out of the ovaries into the coelom are picked up by the 

 somewhat trumpet shaped extensions around the ostia and carried into 

 the oviduct. In some forms the pronephric tubules and nephrostomes 

 take part in the formation of the ostium tubae and the beginning of the 

 Dviduct ; however, as in all the higher forms, the rest of the oviduct arises 

 by the formation of a groove of the peritoneal membrane close beside 

 the Wolffian duct. This becomes rolled into a tube, the Miillerian duct. 

 In the amniotes the anterior end of the groove does not close, but re- 

 mains open as the ostium tubae. (Fig. 458.) 



It is difficult to trace the successive stages from the most primitive 

 to the most highly developed. By some the condition of the oviduct in 

 the elasmobranchs is regarded as the most primitive. Some contend that 

 we have here a homologous condition a condition resulting from similar 

 primitive structures ; others that it is rather analogous and an example 

 of convergent evolution in that these organs, having been used for similar 

 functions, have come to appear somewhat alike structurally. 



It can be seen how difficult valid comparisons are when we have 

 such varying conditions in the lower, but nevertheless supposedly related 



