80 ANIMAL PHYSIOLOGY. 
blastodermic vesicle (yelk-sac) has vascular villi, which in all 
probability not only serve the purpose of attaching the embryo 
to the uterine wall but derive nourishment, not as in birds, from 
the albumen of the ovum, but directly in some way from the 
uterine wall of the mother. It will be remembered that the 
opossum ranks low in the 
mammalian scale, so that, this 
resemblance is the more signifi- 
cant from an evolutionary point 
of view. 
The term chorion is now re- 
stricted to those regions of the 
subzonal membrane to which 
either the yelk-sac or the allan- 
tois is attached. The former 
zone has been distinguished as 
the false chorion and the latter 
Fia. 89.—Diagram of the foetalmembranesof €@S the true chorion. In the 
the Virginian opossum (Haddon, after Os- . : : 
born). ‘Two villi are shown greatly en. Tabbit the false chorion is very 
larged. The processes of the cells, which ac “ 
hate been eraerenited, doubtless corre- large (Fig. 83), and the (placen- 
spend to the Jrcudopodia deserted °Y tal) chorion very small in com- 
sinus, temalfals 3, subeonal mem parison, but the reverse is the 
sac, ‘The vascular splanchnopleure (ay- case in most mammals, It will 
poblast and mesoblast) is indicated by he noted that in both birds and 
mammals the allantois is a nu- 
tritive organ. Usually the more prominent and persistent the 
yelk-sac, the less so the allantois, and: vice versa; they are 
plainly supplementary organs. 
The Placenta.—This structure, which varies greatly in com- 
plexity, may be regarded as the result of the union of structures 
existing for a longer or shorter period, free and largely inde- 
pendent of each other. With evolution there is differentiation 
and complication, so that the placenta usually marks the site 
where structures have met and fused, differentiating a new 
organ; while corresponding atrophy, obliteration, and fusion 
take place in other regions. 
All placentas are highly vascular, all are villous, all dis- 
charge similar functions in providing the embryo with nourish- 
ment and eliminating the waste of its cell-life (metabolism). 
In structural details they are so different that classifications of 
mammals have been founded upon their resemblances and dif- 
ferences. These will now be briefly described. 
In marsupials the yelk-sac is both large and vascular; the 
