contrary, in the mammals, the chorion, which corresponds to the vitelline membrane, is vivified, and finally becomes attached to the maternal body, thus establishing a direct connection between the young and the mother; a connection which is again renewed in another mode, after birth, by the process of nursing.

similar to those described in birds: its body and its organs are formed in the same way; an amnios encloses it, and an allantois grows out of the lower extremity of the little animal. As soon as the allantois has surrounded the embryo, its blood vessels become more and more numerous, so as to extend into the

fringes of the chorion, (Fig. 131, pe;) while,

on the other hand, similar vessels from the ww Te mother extend into the corresponding

fringes of the matrix, (p m,) but without

directly communicating with those of the chorion. These two sorts of fringes soon become interwoven, so as to form an intricate organ filled with blood, called the placenta, to which the embryo remains suspended until birth.

Fig. 131.

315 f. From the fact above stated, it is clear that there are three modifications of embryonic development among vertebrated animals, namely, that of fishes and naked reptiles, that of scaly reptiles and birds, and that of the mammals, which display a gradation of more and more complicated adaptation. In fishes and the naked reptiles, the germ simply encloses the yolk, and the embryo rises and grows from its upper part. In the scaly reptiles and birds there is, besides, an amnios arising from the peripheric part of the embryo and an allantois growing out of the lower cavity, both enclosing and protecting the germ.

SECTION III.

ZOOLOGICAL IMPORTANCE OF EMBRYOLOGY.

318. As a general result of the observations which have been made, up to this time, on the embryology of the various classes of the Animal Kingdom, especially of the verte brates, it may be said, that the organs of the body are successively formed in the order of their organic importance, the most essential being always the earliest to appear. In accordance with this law, the organs of vegetative life, the intestines and their appurtenances, make their appearance subsequently to those of animal life, such as the nervous system, the skeleton, &c.; and these, in turn, are preceded by the more general phenomena belonging to the animal as such.

319. Thus we have seen that, in the fish, the first changes relate to the segmentation of the yolk and the formation of the germ, which is a process common to all classes of animals. It is not until a subsequent period that we trace the dorsal furrow, which indicates that the forming animal will have a double cavity, and consequently belong to the division of the vertebrates; an indication afterwards fully confirmed by the successive appearance of the brain and the organs of sense. Later still, the intestine is formed, the limbs become evident, and the organs of respiration acquire their definite form, thus enabling us to distinguish with certainty the class to which the animal belongs. Finally, after the egg is hatched, the peculiarities of the teeth, and the shape of the extremit es, mark the genus and species.

320. Hence the embryos of different animals resemble each otner more strongly when examined in the earlier stages of the r growth We have already stated that, during

almost the whole period of embryonic life, the young fish and the young frog scarcely differ at all, (313) so it is also with the young snake compared with the embryo bird. The embryo of the crab, again, is scarcely to be distinguished from that of the insect; and if we go still further back in the history of development, we come to a period when no appreciable difference whatever is to be discovered between the embryos of the various departments. The embryo of the snail, when the germ begins to show itself, is nearly the same as that of a fish or a crab. All that can be predicted at this period is, that the germ which is unfolding itself will become an animal; the class and the group are not yet indicated.

321. After this account of the history of the development of the egg, the importance of Embryology to the study of systematic Zoology cannot be questioned. For evidently, if the formation of the organs in the embryo takes place in an order corresponding to their importance, this succession must of itself furnish a criterion of their relative value in classification. Thus, those peculiarities that first appear should be considered of higher value than those that appear later. In this respect, the division of the Animal Kingdom into four types, the Vertebrates, the Articulates, the Mollusks, and the Radiates, corresponds perfectly with the gradations displayed by Embryology.

322. This classification, as has been already shown, (61,) is founded essentially on the organs of animal life, the nervous system and the parts belonging thereto, as found in the perfect animal. Now, it results from the above account, that in most animals the organs of animal life are precisely those that are earliest formed in the embryo; whereas those of vegetative life, on which is founded the division into classes, orders, and families, such as the heart, the respiratory apparatus, and the jaws, are not distinctly formed until after

wards. Therefore a classification, to be true and natural, must accord with the succession of organs in the embryonic development. This coincidence, while it corroborates the anatomical principles of Cuvier's classification of the Animal Kingdom, furnishes us with new proof that there is a general plan displayed in every kind of development.

323. Combining these two points of view, that of Embryology with that of Anatomy, the four divisions of the Animal Kingdom may be represented by the four figures which are to be found, at the centre of the diagram, at the beginning of the volume.

324. The type of Vertebrates, having two cavities, one above the other, the former destined to receive the nervous system, and the latter, which is of a larger size, for the intestines, is represented by a double crescent united at the centre, and closing above, as well as below.

325. The type of Articulata, having but one cavity, growing from below upwards, and the nervous system forming a series of ganglions, placed below the intestine, is represented by a single crescent, with the horns directed upwards.

326. The type of Mollusks having also but one cavity, the nervous system being a simple ring around the esophagus, with ganglions above and below, from which threads go off to all parts, is represented by a single crescent with the horns turned downwards.

327. Finally, the type of Radiata, the radiating form of which is seen even in the youngest individuals, is represented by a star.

CHAPTER ELEVENTH.

PECULIAR MODES OF REPRODUCTION.

SECTION I.

GEMMIPAROUS AND FISSIPAROUS REPRODUCTION.

328. WE have shown in the preceding chapter, that ovulation, and the development of embryos from eggs, is cominon to all classes of animals, and must be considered as the great process for the reproduction of species. Two other modes of propagation, applying, however, to only a limited number of animals, remain to be mentioned, namely, gemmiparous reproduction, or multiplication by means of buds, and fissiparous reproduction, or propagation by division; and also some still more extraordinary modifications yet involved in much obscurity.

329. Reproduction by buds occurs among the polyps, medusæ, and some of the infusoria. On the stalk, or even on the body of the Hydra, (Fig. 132,) and of many infusoria, there are formed buds, like those of plants. On close examination they are found to be young animals, at first very imperfectly formed, and communicating at the base with the parent body, from which they derive their nourishment. By degrees, the animal is developed; in most cases, the tube by which it is connected with the parent