36 OF LIFE, AND ITS CONDITIONS. 



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 pro- 

 cess ; 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 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 prob- 

 ably for the most part returned (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 than it does in the Chick in ovo. Indeed, it is impossible to con- 

 ceive of the growth of any of the higher organisms which not merely con- 

 sists in the formation of new parts, but also involves a vast amount of inter- 

 stitial change withoutperceivingthatin the remodelling which is incessantly 

 going on, the parts first formed must be removed to make way for those 

 which have to take their place. And such removal can scarcely be accom- 

 plished without a retrograde metamorphosis, which, as in the numerous cases 

 already referred to, may be considered with great probability as setting free 

 constructive force to be applied in the production of new tissue. 



17. If, now, we pass oil from the intrauteriue life of the Mammalian 

 organism to that period of its existence which intervenes between birth and 

 maturity, we see that a temporary provision is made in the acts of lactation 

 and nursing for affording both food and warmth to the young creature, 

 which is at first incapable of adequately providing itself with aliment, or of 

 resisting external cold without fostering aid. And we notice that the off- 

 spring of Man remains longer dependent upon parental care than that of 

 any other Mammal; in accordance with the higher grade of development to 

 be ultimately attained. But when the period of infancy has passed, the 

 child that is adequately supplied with food, and is protected by the clothing 

 which makes up for the deficiency of other tegumentary covering, ought to 

 be able to maintain its own heat, save in an extremely depressed tempera- 

 ture; and this it does by the metamorphosis of organic substances, partly 

 derived from its own fabric, and partly supplied directly by the food, into 

 binary compounds. During the whole period of growth and development, 

 we find the producing power at its highest point; the circulation of blood 

 being more rapid, and the amount of carbonic acid gas generated and thrown 

 oft' being much greater in proportion to the bulk of the body, than at any 

 subsequent period of life. We find, too, in the large amount of other excre- 

 tions, the evidence of a rapid metamorphosis of tissue ; and it can hardly 

 be questioned (if our general doctrines be well founded) that the construc- 

 tive force which operates in the completion of the fabric will be derived in 

 part from the heat so largely generated by chemical change, and in part 

 from the descent which a portion of the fabric itself is continually mak- 

 ing from the higher plane of organized tissue to the lower plane of dead 

 matter. This hiirh measure of vital activity can only be sustained by an 

 ample supply of Food ; which thus supplies both innterinl for the construc- 

 tion of the organism, and the force by whose agency that construction is 

 accomplished. How completely dependent the constructive or organizing 

 process still is upon Heat, is shown by the phenomena of reparation in cold- 

 blooded animals ; since not only can the rate at which they take place be 

 experimentally shown to bear a direct relation to the temperature to which 



