Cuap. II. THE WHOLE EGG IS THE EMBRYO. 531 
heart of Vertebrates, while a simple, straight tube,— without even smooth internal 
walls, and while the yolk cells are still dropping from its sides into its cavity and 

only move backwards and forwards like an advancing and retreating tide, — were 
not functionally and typically as fully a heart as later, when it has obtained two 
chambers in Fishes, three in Reptiles, and four in Birds and Mammals; or the lungs, 
while a simple, cul-de-sac-like dilatation of the wall of the cesophagus, were not 
truly performing their part as well as in the Fish when they become more iso- 
lated as a swimming bladder, or in Lepidosteus when they approach the compli- 
cated structure exhibited in Saurians and Chelonians, or in the latter two when 
they occupy a great portion of the cavity of the body, or in Mammals when they 
have changed into a uniform, spongy mass of minute bronchioles with their cap- 
illaries. And so we might mention the progressive stages of the eye, the ear, the 
brain, and all the other organs, if so many examples were necessary. 
At no time is the whole type exemplified by any particular specimen; nor 
does any one individual, at the moment when we look at it, reveal to us its 
whole life. Still less can any alcoholic preparation of an animal, as it hangs inani- 
mate before us, disclose its action, its manner of life, its physical relations, its for- 
mer embryonic simplicity, its later metamorphoses, or its final mode of passing away. 
Such objects ought only to be considered as means for our study, as memo- 
rials of past life. We collect them, that they may assist us in telling the tale 
of their organic connections. We may even substitute wax models for the things 
themselves, and that too with very good success, so far as a plastic substance 
may represent the appearance of animal life at a given period; but the wax is a 
perfect blank as regards the past or the future, and so is the dead animal, when 
compared to what it has been, or to what it might have become. But when 
alive, we see in it at the beginning, as an embryo, certain characteristics of ‘its 
type; when born it exhibits other characteristics, some of the former disappear- 
ing, and some remaining throughout life, and again at various periods of its life 
other characters appear and disappear, so that some individuals, dying before a 
certain age, never wholly exemplify their whole type, whether it be that of branch, 
class, order, family, genus, or species. 
When such views are adopted, and such interpretations have become our stand- 
ards, it is impossible to hold longer to the inanimate nature of any one portion 
of a growing body, and consider the others as endowed with all the characteristics 
of an animate being: it is impossible to assert, if we may revert more specially 
to what has already been said, that the so-called yolk sac is a mere bag of 
nourishment, a reservoir of food, for the embryo, which increases in bulk as the 
former doles out its supplies. What part this organ plays in the progress of 
the growth of Testudinata will be fully described in a future section; let it 
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