650 ZOOLOGY. 
develops in a specialized portion of the oviducts, the uterus 
or womb, and that the growing germ until birth is supplied 
not with yolk as food, but by the nourishment in the ma- 
ternal blood. In fact, while the eggs of reptiles and birds 
are enormous, it was not known with certainty until 1827 
that mammals developed from eggs. The eggs of these an- 
imals are very minute, owing in part to the minute amount 
of yolk they contain ; that of man being less than a quarter 
of a millimetre (4, inch) in diameter. 
The mammalian embryo, nourished as it is through the 
maternal circulation, needs additional temporary organs ; 
these are the chorion (Fig. 541, ch), formed from the vitelline 
membrane (present in birds as well as mammals), which sends 
off villi or processes extending into the walls of the womb. 
Besides this, in the higher or placental mammals, the pla-. 
centa or after-birth is formed, which serves as an organ of 
respiration as well as to supply the embryo or fetus with 
nourishment, and to carry off its effete products by means 
of the maternal circulation. 
It is comparatively late in embryonic life that the mam- 
malian features appear; in the dog it is twenty-five days 
before it can be told whether the embryo is a mammal or 
not. 
All mammals may be said to pass through a morula and 
gastrula stage. In the next stage when the nervous chord 
and notochord arise, the mammalian germ is on the same 
footing with an Ascidian larva. In a succeeding stage, 
when the protovertebre appear, an Amphioxus stage is 
reached ; when a brain is formed, the level of the fishes is 
reached ; after the limbs bud out the young mammals may 
be said to assume the condition common to the embryos of 
all Amphibian and higher Vertebrates. When the allantois 
begins to appear the amphibian feature (the want of an 
allantois) is dropped. When the placenta has developed 
the avian characters are surpassed and the mammalian feat- 
ures assumed. ‘Thus the development of the individual 
mammal is an epitome of that of the branch or type to 
which it belongs, and the successive steps in the degree of 
specialization of the individual mammal are also paralleled 
