REPRODUCTION IN ANIMALS 207 



Throughout all the chief Vertebrate groups the sexes are 

 distinct, although in rare instances abnormal hermaphro- 

 ditic individuals occur. The definitive primordial germ cells 

 first appear as localized areas of the coelomic epithelium, on 

 either side of the vertebral column. As the germ cells develop 

 they become associated with connective tissue, blood vessels, 

 and nerves and form the paired gonads. In the most primi- 

 tive Vertebrates a condition more simple than in the Earth- 

 worm is found, for both male and female germ cells when ripe 

 merely break out of the gonads and find their way to the ex- 

 terior by a pair of minute ABDOMINAL PORES. In higher 

 forms, however, the labor of conducting the products out of 

 the body is foisted upon the urinary system, as was 

 suggested when that system was under discussion. We 

 now turn to a statement of the structural inter-relations 

 of these two systems to form the UROGENITAL SYSTEM. 



It has been pointed out that the nephridia, which combine 

 to form the kidneys, in some of the lower Vertebrates retain 

 their funnel-like openings into the coelom and therefore afford 

 a direct exit for waste material in the coelomic fluid. It is 

 some of these nephridia which are employed in the lower 

 Fishes for the transfer of the germ cells to the outside. The 

 testes of the male, which lie close to the kidneys, become con- 

 nected with the nephridia (mesonephros) by a series of short 

 delicate tubes. Through these tubes the SPERMATIC FLUID, 

 containing the sperm from the testes, is transferred to the 

 nephridia and by them to the kidney (mesonephric) ducts 

 and so to the exterior with the urinary waste. In this way, 

 during the period of sexual activity of the male, the kidney 

 tubules satisfactorily perform two functions, and the mesone- 

 phric ducts become UROGENITAL CANALS. (Fig. 97, C.) 



Turning to the female, we find that the ovaries, which are 

 situated in about the same position with relation to the kid- 



