OP ANIMALS. 41 



only in those animals which have a circulation. Among these, 

 some are hermaphrodite, or are provided with both male and 

 female organs; but this hermaphrodite state is incomplete, or 

 rather insufficient; for in order to engender they require a 

 reciprocal copulation with another similar individual: such 

 is the case with some annelides and mollusca. In a more ele- 

 vated order of beings, the genital organs are separated and 

 borne by different individuals, and this constitutes the sexes. 

 This occurs in some intestinal worms, in many mollusca, in- 

 secta, Crustacea, arachnida, and in all the vertebrate animals. 



25. In sexual generation, the germ is enclosed with nu- 

 tritive substances in a membranous or more solid, and even 

 calcarious envelope ; then it is called an egg. Sometimes the 

 egg contains nutritive materials in a sufficient quantity to de- 

 velope completely the embryo, and receives through its cover- 

 ings the influence of atmospheric air only, and scarcely that of 

 humidity ; the animal is then said to be oviparous, either if the 

 egg be laid entire, and the development of the embryo occur 

 after being laid, or if the development precede the laying of 

 the egg, and the egg break at the moment of its birth. In 

 oviparous generation, the germ is only separated, generally, 

 after fecundation; nevertheless, in some instances the germ is 

 separated before, and the egg is fecundated during or even 

 after the laying. The egg does not always contain sufficient 

 materials for the development of the embryo; in that case it 

 ingrafts itself by its surface in the uterus, and absorbs therein 

 nutritive substances; the young one is born living with the 

 remains of its membranous egg, but in a state of weakness 

 which requires to be nourished with an animal fluid that the 

 mother secretes the milk. The mammalia are alone in this 

 situation. Some young animals, on quitting the egg bear no 

 resemblance to their parent; they experience before reaching 

 their form a change which is called metamorphosis: such are 

 the larva of insects, and the tadpole of the batrachia; the 

 others, on the contrary, are born similar to their parents, or 

 at least there are only some slight differences of proportion, 

 which in time disappear. 



26. Nutrition and generation are not the only two modes 



