DYNAMICS OF EMBRYONIC DELELOPMENT. 427 



nance of the embryo, to the condition of simple binary com- 

 pounds, is an essential condition of the process ; and since it 

 can scarcely be supposed that the object of this metamor- 

 phosis can be to furnish heat (an ample supply of that force 

 being afforded by the body of the parent), it seems not un- 

 likely 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 

 mammalia, imparts to it a steady temperature equivalent to 

 that of the parent itself; and in all save the implacental or- 

 ders of this class, that development is carried still further 

 than in birds, the new-born mammal being yet more com- 

 plete in all its parts, and its size bearing a larger proportion 

 to that of its parent, than even in birds. It is doubtless ow- 

 ing 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 imbibition of these 

 takes the place, during the whole remainder of foetal life, of 

 the appropriation of the materials supplied in the bird's egg 

 by the " food yolk" and " albumen." To what extent a retro- 

 grade 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 fcetal 



