THE LIFE OF MAMMALS 
amount of material is furnished by the egg, and when this is 
exhausted the embryo is voided, placed in a protective pouch 
and thereafter nourished and increased by milk forced into its 
system. Now in the Prototheria the egg is comparatively large, 
contains much food yolk, and in fact resembles that of a turtle, 
even to the parchmentlike shell; and it is laid and brooded in a 
manner very similar to that of some sea birds, especially the 
penguins, until the embryo has utilized all its contents and is 
ready to come out — not as perfect as a young penguin would 
be, but far more advanced than a marsupial “larva.” 
This process, then, is outside of the established order of 
mammalian reproduction, but is remarkable only because of 
its resemblance to the methods of the inferior vertebrates. 
Hence it is an indication of low and “ generalized”’ organization 
and rank in the mammalian scale. Further indication of this 
is given in the fact that when the embryos come out of the eggs 
they are not able, as are infant reptiles, birds, and the like, to 
take general food, but must for a time subsist on milk like other 
mammals. The provision for nursing, however, in this group, 
is very imperfect; and for a full explanation of it such text- 
books as Parker and Haswell’s “Zodlogy’’? should be consulted. 
Briefly, instead of the milk-secreting glands opening at the sur- 
face in teats the skin of that part of the abdomen overlying 
them is thin and bare and shows no nipples at all; at the time 
when the young arrive this area will yield milk at two points in- 
dicated by tufts of hair, which presumably guide the sensitive 
nose of the half-witted youngster to the right place at which to 
suck. 
As might be expected, all the Prototheria arc Australian; nor 
is there evidence of their ever having existed in any other part 
of the world, unless it be conceded that the Mesozoic “ multi- 
tuberculates”’ were their ancestors. 
Existing representatives of this remarkable group are few, — 
only the duckbill and the spiny ant-eaters. 
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