45° 



ZOOLOGY 



of the embryo is developed in the same manner as its other tissues 

 and is not derived from the mother directly. The two blood 

 currents interchange materials by osmosis through the capillary 

 walls. There is no direct continuation of nervous tissue across 

 from the mother to the young. Notwithstanding this lack of 

 direct connection each exerts a very profound effect upon the 

 other. It is known that the embryo secretes materials which 

 passing into the blood of the mother produce the growth of the 

 mammary glands that leads finally to lactation. Proper nutri- 

 tion, malnutrition, endocrines, or poisons in the blood of the 

 mother greatly influence the growth and normality of the 

 offspring. 



463. Classification of Mammals.— In the introductory sur- 

 vey in §448 the three subclasses have already been outlined. 

 Stibclass I. Prototheria, Order Rloiiotrcmata. — Mammals 



Fig. 238. 



Fig. 238. Dnck-hill {Ornithorhynchus analinus). Photographed by Folsom. 



Questions on the figure. — What are the peculiarities of Ornithorhynchus? 

 What does the structure of its feet indicate as to its habits? 



whose mammary glands have no nipples; they lay eggs with 

 abundant yolk, which are hatched outside the body, as in birds. 

 The alimentary canal ends in a cloaca. One ovary is sometimes 

 incompletely developed as in birds, and the oviducts open 

 separately into the vagina. The duck-bill or duck-mole 



