86 ANIMAL PHYSIOLOGY. 
in the blood of the large irregular sinuses of the decidua sero- 
tina; but this is so unlike what prevails in other groups of 
animals that we can not refrain from believing that the state- 
ment is not wholly true. 
The Zonary Placenta.—In this type the placenta is formed 
along a broad equatorial belt, leaving the poles free. This form 
of placentation is exemplified in the carnivora, hyrax, the ele- 
phant, etc. 
In the dog, for example, the yelk-sac is large, vascular, does 
not fuse with the chorion, and persists throughout. A rudiment- 
ary discoid placenta is first formed, as in the rabbit; this grad- 
ually spreads over the whole central area, till only the extremes 
(poles) of the ovum remain free; villi appear, fitting into pits 
in the uterine surface, the maternal and foetal parts of the pla- 
centa becoming highly vascular and closely approximated. 
The chorionic zone remains wider than the placental. As in 
man there is at birth a separation of the maternal as well as 
foetal part of the placenta—i. e., the latter is deciduate; there is 
also the beginning of a decidua reflexa. 
The Diffuse Placenta.—As found in the horse, pig, lemur, etc., 
‘the allantois completely incloses the embryo, and it becomes 
villous in all parts, except a small area at each pole. 
' The Polycotyledonary Placenta.—This form is that met with in 
ruminants, in which case the allantois completely covers the 
surface of the subzonal membrane, the placental villi being 
gathered into patches (cotyledons), which are equivalent to so 
many independent placentas. The component villi fit into cor- 
responding pits in the uterine wall, which is specially thickened 
at these points. When examined in a fresh condition, under 
water, they constitute very beautiful objects. 
Comparing the formation, complete development, and atro- 
phy (in some cases) of the various foetal appendages in mam- 
mals, one can not but perceive a common plan of structure, 
with variations in the preponderance of one part over another 
here and there throughout. In birds these structures are sim- 
pler, chiefly because less blended and because of the presence 
of much food-yelk, albumen, egg-shell, etc., on the one hand, 
and the absence of a uterine wall, with which in the mammal 
the membranes are brought into close relationship, on the other ; 
but, as will be shown later, whatever the variations, they are 
adaptations to meet common needs and subserve common ends, 
