18 INTRODUCTION. 
others it oozes from the bottom of little cavities. Before these arterial 
extremities change into veins, they most commonly give rise to particular 
vessels that convey this fluid, which appears to proceed from the exact 
point of union between the two kinds of vessels; in this case the blood 
vessels and these latter form, by interlacing, particular bodies called con- 
glomerate or secretory glands. 
In animals that have no circulation, in insects particularly, the parts 
are all bathed in the nutritive fluid: each of these parts draws from it 
what it requires, and if the production of a liquid be necessary, proper ves- 
sels floating in the fluid take up, by their pores, the constituent elements of 
that liquid. 
It is thus that the blood incessantly supports the composition of all the 
parts, and repairs the injuries arising from those changes which are the 
continual and necessary consequences of their functions. The general 
ideas we form with respect to this process are tolerably clear, although we 
have no distinct or detailed notion of what passes at each point; and for 
want of knowing the chemical composition of each part with sufficient 
precision, we cannot render an exact account of the transmutations neces- 
sary to effect it. 
Besides the glands which separate from the blood those fluids that are 
destined for the internal economy, there are some which detach others 
from it that are to be totally ejected, either as superfluous—the urine, for 
instance, which is produced by the kidneys; or for some use to the animal— 
as the ink of the cuttle-fish, and the purple matter of various mol-* 
lusca, &c. 
With respect to generation, there is a process or phenomenon, infinitely 
more difficult to comprehend than that of the secretions—the production 
of the germ. We have even seen that it is to be considered as almost in- 
comprehensible; but the existence of the germ being admitted, generation 
presents no particular difficulties. As long as it adheres to the parent, it 
is nourished as if it were one of its organs; and when it detaches itself, it 
possesses its own life, which is essentially similar to that of the adult. 
The germ, the embryo, the foetus, and the new-born animal have never, 
however, exactly the same form as the adult, and the difference is some- 
times so great, that their assimilation has been termed a metamorphosis. 
Thus, no one not previously aware of the fact, would suppose that the ca- 
terpillar is to become a butterfly. 
Every living being is more or less metamorphosed in the course of its 
growth; that is, it loses certain parts, and developes others. ‘The anten- 
ne, wings, and all the parts of the butterfly were inclosed beneath the skin 
of the caterpillar; this skin vanishes along with the jaws, feet, and other 
organs, that do not remain with the butterfly. The feet of the frog are 
