478 OF FOOD, AND THE DIGESTIVE PROCESS. 



is invariably followed in the development of the Animal ; the nutriment stored 

 up in the ovum being usually sufficient for the evolution of the fabric, until it 

 acquires the power of ingesting food for itself; and where this is not the 

 case (as in the Mammalia), a further provision being adopted, by which the 

 supply is continued during a lengthened period. Even when thrown upon 

 its own resources, the young Animal is often far from having attained even 

 the form of its parent; much less its size; and in the progress of its evolu- 

 tion, a greater or less degree of metamorphosis or change of form is observa- 

 ble. This is not usually so much the case in the higher animals, as in the 

 lower; because the supply of nutriment is proportionally greater in the 

 former, and serves to carry on the development to a later period ; but the 

 changes of condition which their germinal structure undergoes within the 

 ovum, are really as remarkable as those which are presented in the early em- 

 bryos of the latter after their emersion from the egg. 



a. The phenomena of metamorphosis are most familiarly known in the case of Insects, 

 and Frogs, which were formerly thought to be exceptions to all general rules ; the Insect 

 coming forth from the egg in the state of a Worm; and the Frog in the condition of a Fish. 

 But it is now known that changes of form, as complete as these, occur in a large proportion 

 of the lower tribes of Animals ; so that the absence of them is the exception. The true 

 mode of viewing these early aspects of Animals of the inferior groups, seems to be to re- 

 gard them as foetal or embryonic ; thus, the Insect, in its larva state, is essentially a fcetus, 

 as regards the grade of development of its several tissues and organs; but it is a foetus 

 capable of obtaining its own nourishment. In this condition it attains its full growth as re- 

 gards size, though its form remains the same; but it then, in passing into the Chrysalis state, 

 re-assumes (as it were) the condition of the embryo within the egg, the development of 

 various new parts takes place, at the expense of the nutriment stored up in its tissues, 

 and it comes forth in the state of the perfect Insect, which henceforth takes no more food 

 than is requisite for the maintenance of the fabric thus evolved, or for the preparation of the 

 stores to be imparted to the offspring. In many of the lower tribes, the animal quits the 

 egg at a still earlier period in comparison ; thus it has been lately shown by M. Milne Ed- 

 wards, that some of the long Marine Worms consist only of a single segment, forming a 

 kind of head, when they leave the egg ; and that the other segments, to the number it may 

 be of several hundred, are gradually developed from this ; the evolution continuing in some 

 instances during a considerable part of life. In some of the Radiated tribes, propagation 

 actually takes place whilst the animal is yet in its first or imperfect form ; thus the- Meduste 

 begin life as Polypes, and in this condition they increase by germination or budding, in the 

 manner of the true or permanent Polypes. 



631. It is desirable to bear in mind, that the function of the Germ is sim- 

 ply that of occasioning the combination of the materials supplied by the 

 external world, and of directing the appropriation of those materials. The 

 several parts of the complex fabric of the higher Animals, contain a great 

 variety of materials ; and it is therefore requisite for its development, that it 

 should be duly supplied with all these. The demand set up by the fabric, 

 whilst in course of development or evolution, for the materials of its growth, 

 constitutes, therefore, the primary source of the requirement of food ; and tin- 

 nature of this must be adapted to the wants of the being. Thus, the fabric 

 of Plants is essentially composed of Cellulose, a compound of Oxygen, Hy- 

 drogen, and Carbon ; and the materials required for the production of this are 

 simply Carbonic Acid and Water. But. nearly all Plants form some azotized 

 compound in the interior of their cells ; for the production of which, Ammo- 

 nia also is required. And in those species, whiclis like the Ccrealia, form a 

 large quantity of azotized compounds, and store them up in their seeds, a free 

 supply of Ammonia is requisite for the production of the greatest proportion 

 which they are capable of generating. In Animals, again, whose tissue chiefly 

 consists of these very azotized compounds, or of modifications of them, a 

 constant supply of such is required during the whole period of the develop- 

 ment of the fabric, as well as subsequently; and if they be not afforded in 



