FCETAL ENVELOPES 379 



cloaca. As stated above, in all other mammals the connexion of 

 the urogenital sinus with the cloaca is lost in the early stages, though 

 in the lower groups the capacity of protrusion and retraction of the 

 penis is retained. 



In both sexes of the other mammals the same anlagen of the external genitalia 

 are found. These consist of the genital prominence already mentioned, which 

 is formed from the ventral or anterior wall of the cloaca. This soon protrudes 

 from the opening and, when the perineum is formed, two thickenings appear on 

 either side, a more medial genital fold and a larger and lateral genital ridge, 

 these extending back nearly to the level of the anus (fig. 403). In the female the 

 genital prominence never develops much farther, while the folds and ridges 

 become the labia minora and majora. In the male a groove is formed on the 

 primitively dorsal surface of the prominence, this continuing into the cloaca. 

 Then the folds grow together behind the prominence, closing the groove to a tube, 

 the urethra, while the prominence becomes the glans penis. A similar growth 

 of the genital ridges toward the median line results in the formation of the 

 outer wall of the scrotum. 



In the lower mammals there is the same retractile condition of the 

 penis as in the monotremes (figs. 401, 403, //), but in the higher 

 groups it is permanently exserted (fig. 403, ///). In the marsupials 

 the tip of the penis is frequently bifurcate, corresponding to the two 

 vaginae of the female. In many rodents, bats, many carnivores, 

 whales and a few primates a penis bone is developed in the middle line 

 of the intromittent organ (fig. 401) . In all there are the same corpora 

 fibrosa and cavernosa as in the sauropsida. 



NUTRITION AND RESPIRATION OF THE 

 EMBRYO-FCETAL ENVELOPES 



In all vertebrates except the mammals there is enough nourish- 

 ment stored in the egg to carry the young through its development 

 up to the point where it hatches and shifts for itself. In the cyclo- 

 stomes, dipnoi and amphibia this nourishment (food-yolk or deuto- 

 plasm) is soon enclosed by the body wall. In ganoids and teleosts, 

 where it is relatively larger in amount, it forms for a time a projecting 

 mass enclosed in a yolk sac, and this condition reaches it extreme in 

 the elasmobranchs and sauropsida. The yolk sac, in the fishes, is an 

 extension of the intestine and the body wall, and is richly supplied by 

 vitelline arteries and veins which are derivatives of the omphalo- 

 mesenteric vessels (p. 309). In the sauropsida, owing to the develop- 

 ment of the amnion and the consequent separation of the non- 

 embryonic somatopleure from the yolk, the yolk sac is composed of 



