THE URO-GEXITAL SYSTEM 369 



furnishes an investment and suspensory ligament for the in- 

 testine. The ccclom is formed, as has been shown, by the ex- 

 pansion and later fusion of numerous pairs of gonadic sacs, 

 and is thus an expanded gonadic cavity, while the peritoneum- 

 is identical with the walls of a great compound gonadic 

 sac. The many pairs of gonadic openings are lost and appear 

 either as a single pair (the pori abdominalis of cyclostomes 

 and selachians) or are entirely lost (all higher vertebrates). 

 Owing to this reduction either the second method for the 

 liberation of germ-cells is employed, the utilization of ne- 

 phridia, or else secondary ducts are developed to serve the 

 purpose. The proliferating masses of germ-cells project from 

 the peritoneal wall and become suspended in band-like liga- 

 ments like the mesenteries of the intestine, the mesovarium or 

 mesorchium, and may either remain in their original loca- 

 tion or become displaced and assume a secondary position. 

 Finally the nephridia become confined to a certain region of 

 the body, where they may form a pair of single definite masses, 

 the kidneys, the units of which no longer open externally by 

 independent openings, but become attached to a common ex- 

 current duct, which opens, either independently, or, more 

 usually, into the terminal portion of the alimentary canal. 



Turning now from theory to fact, we may take up the uro- 

 genital organs as they actually exist in the various vertebrate 

 groups, thus tracing a history by means of which the con- 

 dition found in Man may receive at least a partial explanation. 

 The condition in the cyclostomes is difficult to interpret; the 

 teleosts seem not to come into line with the rest and represent 

 a side branch, which, perhaps, presents an independent so- 

 lution of the problem ; but from the selachians and certain ga- 

 noids directly to the amphibians, and from them to the Am- 

 niota the history is a fairly continuous one. For the sake of 

 clearness it will be best to consider separately the two systems 

 involved, beginning with the urinary. 



\Ye have already learned that organs performing the same 

 function in the different groups of vertebrates are not neces- 

 sarily homologous, and are familiar with such phenomena as 



