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 zat.) 
Ureter. Kidney. 
Epididymis, 
Vas Deferens, 
Urogenital 
Sinus, 
Rectum, 
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 
