24 INTRODUCTION. 
With respect to generation, there is a process or phenome- 
non, infinitely more diflicult to comprehend than that of the 
secretions—the production of the germ. We have even seen 
that it is to be considered as almost incomprehensible; 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 de- 
taches itself, it possesses its own life, which is essentially simi- 
lar to that of the adult. 
The germ, the embryo, the foetus, and the new-born ani- 
mal have never, however, exactly the same form as the adult, 
and the difference is sometimes so great, that their assimilation 
has been termed a metamorphosis. ‘Thus, no one not previ- 
ously aware of the fact would suppose that the caterpillar 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 deve- 
lopes others. The antennz, wings, and all the parts of the 
butterfly were enclosed beneath the skin of the caterpillar 5 
this skin vanishes along with the jaws, feet, and other organs, 
that do not remain with the butterfly. The feet of the frog 
are enclosed by the skin of the tadpole; and the tadpole, to 
become a frog, parts with its tail, mouth, and branchie. The 
child, at birth, loses its placenta and membranes; at a certain 
period its thymus gland nearly disappears, and it gradually 
acquires hair, teeth, and beard; the relative size of its organs 
is altered, and its body augments in a greater ratio than its 
head, the head more than the internal ear, &c. 
The place where these germs are found, and their germs 
themselves are collectively styled the ovary; the canal through 
which, when detached, they are carried into the uterus, the 
oviduct; the cavity In which, im many species, they are com- 
pelled to remain for a longer or shorter period previous to 
birth, the wferus ; and the external orifice through which they 
pass into the world, the vulva. Where there are sexes, the 
male impregnates, the germs appearing in the female. The 
fecundating liquor is called semen; the glands that separate 
he oe | 
