1864.] Carrrentur on Correlation of Physical and Vital Forces. 263 
which is needed in the case of cold-blooded animals; and we may 
notice two results of this application as very significant of the 
dynamical relation between Heat and Developmental Force,—first, 
that the period required for the evolution of the germ into the mature 
embryo is nearly constant, each species having a definite period of 
incubation,—and second, that the grade of development attained by 
the embryo before its emersion is relatively much higher than it is in 
cold-blooded Vertebrata, generally; the only instances in which 
anything like the same stage is attained without a special incubation, 
being those in which (as in the Turtle and Crocodile) the eggs are 
hatched under the influence of a high external temperature. This 
higher development is attained at the expense of a much greater 
consumption of nutrient material; the store laid up in the “food yolk” 
and “ albumen” of the Bird’s egg being many times greater in propor- 
tion to the size of the animal which laid it, than that contained in the 
whole egg of a Frog or a Fish. There is evidence in that liberation 
of carbonic acid which has been ascertained to go on in the egg (as 
in the germinating seed) during the whole of the developmental 
process, that the return of a portion of the organic substances pro- 
vided for the sustenance of the embryo, to the condition ‘of simple 
binary compounds, is an essential condition of the process ; and since 
it can scarcely be supposed that the object of this metamorphosis can 
be to furnish heat (an ample supply of that force being afforded by 
the body of the parent), it seems not unlikely that its purpose is to 
supply a force that concurs with the heat received from without in 
maintaining the process of organization. 
The development of the embryo within the body, in the Mam- 
malia, imparts to it a steady temperature equivalent to that of the 
parent itself ; and in all save the implacental Orders of this class, that 
development is carried still further than in Birds, the new-born Mam- 
mal being yet more complete in all its parts, and its size bearing a 
larger proportion to that of its parent, than even in Birds. It is 
doubtless owing in great part to the constancy of the temperature to 
which the embryo is subjected, that its rate of development (as shown 
by the fixed term of utero-gestation) is so uniform. The supply of 
organizable material here afforded by the ovum itself is very small, 
and suffices only for the very earliest stage of the constructive process ; 
but a special provision is very soon made for the nutrition of the 
embryo by materials directly supplied by the parent ; and the imbi- 
bition of these takes the place, during the whole remainder of fcetal 
life, of the appropriation of the materials supplied in the bird’s egg 
by the “food yolk” and “albumen.” To what extent a retrograde 
metamorphosis of nutrient material takes place in the foetal Mammal, 
we have no precise means of determining; since the products of that 
metamorphosis are probably for the most part imparted (through the 
placental circulation) to the blood of the mother, and got rid of 
through her excretory apparatus. But sufficient evidence of such a 
metamorphosis is afforded by the presence of urea in the amniotic 
fluid and of biliary matter in the intestines, to make it probable that 
it takes place not less actively (to say the least) in the foetal Mammal 
T 2 
