THE URO-GENITAL SYSTEM 



397 



These delicate motile cells can exist only in a liquid medium, 

 and from this cause alone arise in all terrestrial animals the 

 necessities, first, of secreting a liquid to serve as a vehicle for 

 the male germ cells, and second, of developing organs through 

 which this liquid may be directly transmitted to the cavities 

 of the female organs, without suffering from the drying action 

 of the air. That this necessity was not immediately apparent 



FIG. in. Diagrams illustrating the descent of the testes in mammals. 

 [After WEBER.] 



t, testis; p, epididymis; s, spermatic cord; m, mesorchium; It, ligamentum testis; 

 /*, ligamentum inguinale; i, bursa inguinalis; x, conus inguinalis; y, chorda guber- 

 naculi ( = ligamentum scroti) ; a, tunica vaginalis propria, visceral layer; b, the same, 

 parietal layer; c, tunica vaginalis communis, continuous with the fascia trans- 

 versa; d, cremaster (=transversalis-obliq. int. abdom.); e, fascia cremasterica Cooped 

 ( obliq. ext. abdom.); f, integument, including the tunica dartos and involuntary 

 muscular layer. 



is due to the semi-aquatic habits of most amphibians, even the 

 most terrestrial of which resort to the water at the breeding 

 season and are thus able to dispense with any external mech- 

 anism; yet here, notwithstanding the absence of external 

 organs, there have arisen numerous habits, such as the love 

 antics of salamanders and the amplexation of frogs and toads, 

 which are designed to secure a greater likelihood of fertiliza- 



