34 INTRODUCTION. 



With respect to generation^ there is a process or phenome- 

 non, 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 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 antennse, wings, and all the parts of the 

 butterfly were enclosed 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 enclosed by the skin of the tadpole ; and the tadpole, to 

 become a frog, parts with its tail, mouth, and branchiae. 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, in many species, they are com- 

 pelled to remain for a longer or shorter period previous to 

 birth, the uterus; 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 



