THE UROGENITAL SYSTEM 407 



Even the ureter of amniotes seems not to be a novelty in the group. 

 In some elasmobranchs, a secondary renal duct arises as an outgrowth 

 from the primitive duct in a manner similar to that of the ureter. A 

 urinary duct, which in the embryos of some reptiles drains the posterior 

 portion of the mesonephros, develops like the ureter of amniotes as an 

 outgrowth of the mesonephric duct. The fact that in some mammals 

 and in man a number of ureters may develop, suggests the possibility 

 that ureters were primitively multiple but that they have become reduced 

 in number in mammals. The present ureter has been regarded also as 

 an elongated renal tubule which secondarily acquires functional con- 

 nexions with the numerous tubules of the metanephros. 



In the female amniote, the mesonephros disappears except in the form 

 of functionless rudiments, the epoophoron, the paroophoron, and the duct 

 of Gartner. In the male, the primitive mesonephric duct is utilized as a 

 ductus deferens. The anterior part of the mesonephros becomes the 

 epidid)nnis. Such relations are inherited from those of anamnia. Rem- 

 nants of the posterior portion of the mesonephros may persist in the adult 

 as the rudimentary paradidymis and ductus aberrans. 



In vertebrates except most mammals, the excretions are poured into 

 the cloaca. This is also true of monotremes. A bladder comparable to 

 that of mammals makes its first appearance in amphibia as a hollow out- 

 growth from the floor of the cloaca. In neither the amphibians nor 

 reptiles, however, is the bladder directly connected with the excretory 

 ducts, so that excretions reach it only by way of the cloaca. In those 

 mammals which are without a cloaca, the ureters acquire direct connexions 

 with the bladder and open upon its dorsal surface. 



The Reproductive System 



Cells speciahzed as reproductive elements make their first appearance 

 in colonial protozoa. Sexual reproduction, however, involving the 

 union of two gametes occurs in all classes of protozoa. Protozoa conju- 

 gate periodically. The beginnings of differentiation of gametes appear 

 also in protozoa. In unicellular organisms transitional stages between 

 the union of similar gametes, isogamy, and that of speciahzed eggs and 

 spermatozoa, heterogamy, may be recognized. Such a differentiation of 

 gametes is generally interpreted as adaptive. The ovum contains the 

 food supply for the developing embryo and consequently loses the motility 

 which is retained by the spermatozoon as a way of insuring union of 

 gametes. Biologists are not agreed in regard to the meaning of sexual 

 reproduction. Some hold that it increases variability, while others 

 assume that it increases stability of species by checking extreme variation. 



Coelenterates. Among coelenterates the individual may be hermaph- 

 roditic as in the case of hydra, or sexually differentiated as in the case of 



