EGG-LAYING MAMMALS — Subclass, PROTOTHERIA : 
Order, MONOTREMATA 
THERE remain to beaccounted for those strange, antique mam- 
mals, the duckbill and echidnas, which stand in a group infe- 
rior to and widely separated from the remainder of the class. Of 
their fossil history no evidences have been discovered beyond 
the most recent past (Pleistocene), yet there can be no doubt 
_ that they represent a very primitive type. Their anatomy, fully 
sketched and discussed in Beddard’s *” and other recent text- 
books, displays many very archaic features, some bearing a 
close resemblance to reptilian modes of structure. There is no 
question of their generally inferior place in classification; and 
hence comes their designation Prototheria (“primitive beasts’’) 
to take the place of the less precise name ‘‘Ornithodelphia.”’ 
They are often spoken of as “ monotremes.”’ 
Their most conspicuous peculiarity lies in the method of 
reproduction : — 
All animals begin as eggs; and in all the lower forms of verte- 
brates (fishes, amphibians, reptiles, and birds) the embryo is 
formed and remains until far advanced in growth 
within the protective envelopes (shell) of the egg, 
which in most cases is dropped from the body of the parent at 
an early stage, and among the higher ranks is thereafter warmed 
and cared for until the embryo is prepared to come forth; and 
this egg contains.a quantity of food material (yolk, etc.). In 
placental mammals, however, the nutriment necessary for the 
growth of the embryo is supplied not by the egg but by the 
mother’s blood. In implacental mammals (marsupials) a small 
521 
Eggs. 
