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the former, the so-called ‘ amnion,’ is a sac filled with fluid, 
which invests the whole body of the embryo, and plays the 
part of a sort of water-bed for it; the other, termed the 
‘allantois,’ grows out, loaded with blood-vessels, from the 
ventral region, and eventually applying itself to the walls of 
the cavity, i which the developing organism is contained, 
enables these vessels to become the channel by which the 
stream of nutriment, required to supply the wants of the off- 
spring, is furnished to it by the parent. 
The structure which is developed by the interlacement of 
the vessels of the offspring with those of the parent, and by 
means of which the former is enabled to receive nourishment 
and to get rid of effete matters, is termed the ‘ Placenta.’ 
It would be tedious, and it is unnecessary for my present 
purpose, to trace the process of development further; suffice 
it to say, that, by a long and gradual series of changes, the 
rudiment here depicted and described, becomes a puppy, is 
born, and then, by still slower and less perceptible steps, 
passes into the adult Dog. 
There is not much apparent resemblance between a barn- 
door Fowl and the Dog who protects the farm-yard. Never- 
theless the student of development finds, not only that the 
chick commences its existence as an egg, primarily identical, 
in all essential respects, with that of the Dog, but that the 
yelk of this egg undergoes division—that the primitive groove 
arises, and that the contiguous parts of the germ are fashioned, 
by precisely similar methods, into a young chick, which, at 
one stage of its existence, is so like the nascent Dog, that 
ordinary inspection would hardly distinguish the two. 
The history of the development of any other vertebrate 
animal, Lizard, Snake, Frog, or Fish, tells the same story. 
There is always, to begin with, an egg having the same essen- 
tial structure as that of the Dog :—the yelk of that egg always 
undergoes division, or ‘segmentation’ as it is often called : the 
ultimate products of that segmentation constitute the building 
