THE MARSUPIALS — Order, MARSUPIALIA 
THE marsupials are a group of mammals so peculiar in their 
structure and distribution that for a long time they were re- 
garded as constituting a subclass completely apart. Their 
most striking peculiarity is their method of reproduction. The 
female’s internal reproductive organs are double throughout, 
whereas in all the mammals heretofore described these organs 
are paired only in the ovaries, from which ducts lead to a single 
sac (uterus) in which the offspring begins its growth. The 
fertilized egg there becomes attached to a particular part of 
the interior wall and develops in organic connection with the 
mother, her blood circulating through the veins of the embryo, 
and nourishing it by means of an intervening temporary modi- 
fication of the surface of the uterus called the ‘‘placenta”’; and 
there the embryo remains growing until it has reached an 
advanced age of readiness. Such mammals are said to be 
“placental.” In marsupials and more primitive monotremes, 
Reproduc- however, the internal reproductive organs of the 
non female are more or less perfectly paired in all their 
parts; and the embryos have no organic attachment to the wall 
of the uterus, so that these mammals are ‘‘implacental.’’ Hence, 
instead of remaining within the mother’s body until they are 
grown into a condition where they can almost take care of 
themselves as soon as born, the embryos in this group escape 
from the uterus and body of the mother at a very early stage, 
when utterly helpless and minute, being, even in the case of the 
largest kangaroos, hardly as big as mice. It would be fatal, of 
course, to turn them loose upon the world; and therefore the 
mother is provided with a fold of skin, forming a more or less 
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