308 INTRODUCTION. 



birds, as we have seen, the nutrition of the developing 

 embryo is entirely effected at the expense of the food- 

 yolk and albumen with which the ovum was charged 

 in the c^'-iry and oviduct respectively, and the eggs 

 leave the parent very soon after the close of segmenta- 

 tion. In the Mammalia the absence of sufficient food- 

 yolk necessitates the existence of some other source of 

 nutriment for the embryo, and that source is mainly the 

 maternal blood. 



The development of Mammalia may be divided into 

 two periods : 1. the development within the uterus ; 2. 

 the development after birth. 



In all the higher Mammalia the second period is very 

 unimportant, as compared with the first ; for the young 

 are born in a condition closely resembling that of the 

 adult of the species to which they belong. The de- 

 velopment during the first period takes place in the 

 uterus of the mother, and nutriment passes from the 

 maternal blood to that of the embryo by means of a 

 structure, to be described in detail hereafter, known as 

 the placenta. This difference between the development 

 of Birds and Mammals may be briefly expressed by saying 

 that the former are oviparous, while the latter are vivi- 

 parous. 



The source of nutriment during the second period 

 is the Mammary glands. In certain of the lower Mam- 

 malia (Marsupials) the young are bom in a very im- 

 mature condition, and become attached by their mouths 

 to the nipples of these glands. They are carried 

 about, usually in a special pouch (marsupium) by the 

 mother, and undergo in this position the greater part of 

 the remainder of their development. 



