MAMMALIA. 489 



those of other mammals, have a tough flexible shell and 

 a large quantity of yolk. The segmentation is meroblastic, 

 like that of reptiles, and the foetal membranes are well 

 developed, as in Sauropsida^ the yolk-sac functions for store 

 of nourishment and the allantois for respiration. The 



Fig- 333. — Ventral View of Male Urogenital Organs 

 OF Ornithorhynchus. {Ad nat.) 



Ureter. Kidney. 



Epididymis. 



Testis. 



Vas Deferens. 



Penis.. 



Cloaca. 



amnion does not appear to completely separate from the 

 serosa, hence the yolk-sac takes up the left and the allantois 

 the right half of the egg-cavity, instead of ventral and dorsal, 

 respectively, as in the chick. 



2. Urogenital Organs. — The base of the oviducts is 

 swollen into a so-called '^uterine" part which probably 



