all the recesses cf the rays; and, in color and structure, re sembles the liver of mollusks. Even in polyps, we find peculiar brown cells lining the digestive cavity, which, probably, perform functions similar to those of the liver in the higher animals.

270. The great importance of the respiratory organs in discharging carbon from the blood has already been spoken of, (245, 251.) The substances removed by the liver and the lungs are of the same class, being those which are desti. tute of nitrogen. These organs seem, in some sense, subsidiary to each other; and hence, in those animals where the respiratory organs are largely developed, the biliary organs are comparatively small, and vice versa. Another and opposite class of impurities, and no less pernicious if retained in the blood, is removed by the KIDNEYS; and, consequently, organs answering to the kidneys are found very far down in the series of animals. Most of the peculiar ingredients of the urine are capable of assuming solid, crys talline forms; and, in some animals, as in reptiles and birds, the whole secretion of the kidneys is solid. In most cases, however, the urinary salts are largely diluted with water; and, as the lungs and liver are supplementary to each other in the removal of carbon, so the lungs, the kidneys, and the skin mutually relieve each other in the removal of the watery portions of the blood.

CHAPTER TENTH

EMBRYOLOGY.

SECTION I

OF THE EGG.

271. THE functions of vegetative life, of which we nave treated in the preceding chapters, namely, digestion, circulation, respiration, and secretion, have for their end the pres ervation of the individual. We have now to treat of the functions that serve for. the perpetuation of the species, namely, those of reproduction, (200.)

272. It has been generally admitted that animals as well as plants are the offspring of individuals of the same kind; and vice versa, that none of them can give birth to individuals differing from themselves; but recent investigations have modified to a considerable extent this view, as we shall see hereafter.

273. Reproduction in animals is almost universally accom plished by the association of individuals of two kinds, males and females, living commonly in pairs or in flocks, each of them characterized by peculiarities of structure and external appearance. As this distinction prevails throughout the animal kingdom, it is always necessary, if we would obtain a correct and complete idea of a species, to take into account the peculiarities of both sexes. Every one is familiar with the differences between the cock and the hen, the lion and the lioness, &c. Less prominent peculiarities are observed in

most. Vertebrates.

Among Articulata, the differences are no less striking, the males being often of a different shape and color, as in crabs, or having even more complete organs, as in many tribes of insects, where the males have wings, while the females are destitute of them, (Fig. 147.) Among mollusks, the females have often a wider shell.

274. Even higher distinctions than specific ones are based upon peculiarities of the sexes; for example, the whole class of Mammalia is characterized by the fact that the female is furnished with organs for nourishing her young with a peculiar liquid, the milk, secreted by herself. Again, the Marsupial, such as the opossum and kangaroo, are distinguished by the circumstance that the female has a pouch into which the young are received in their immature condition at birth.

275. That all animals are produced from eggs, (Omne vivum ex ovo,) is an old adage in Zoology, which modern researches have fully confirmed. In tracing back the phases of animal life, we invariably arrive at an epoch when the incipient animal is enclosed within an egg. It is then called an embryo, and the period passed in this condition is called the embryonic period.

276. Before the various classes of the animal kingdom had been attentively studied during the embryonic period, all animals were divided into two great divisions: the oviparous, comprising those which lay eggs, such as birds, reptiles, fishes, insects, mollusks, &c., and the viviparous, which bring forth their young alive, like the mammalia, and a few from other orders, as the sharks, vipers, &c. This distinction lost much of its importance when it was shown that viviparous animals are produced from eggs, as well as the oviparous; only that their eggs, instead of being laid before the development of the embryo begins, undergo their early changes in the body of the mother. Production from

eggs should therefore, be considered as a universal charac teristic of the Animal Kingdom.

277. Form of the Egg. The general form of the egg is more or less spherical. The eggs of birds have the form of an elongated spheroid, narrow at one end; and this form is so constant, that the term oval has been universally adopted to designate it. But this is by no means the usua form of the eggs of other animals. In most instances, on the contrary they are spherical, especially among the lower animals.

Some

have singular appendages, as those

Fig. 95.

of the skates and sharks, (Fig. 95,) which are shaped like a hand-barrow, with four hooked horns at the corners.

eggs of the hydra, or fresh water polyp, are thickly covered with prickles, (Fig. 96.) Those of certain insects, the Podurella, for example, are furnished with fila

Fig. 96.

Fig. 97.

The

ments which give them a hairy aspect, (Fig. 97;) others are cylindrical or prismatic; and frequently the surface is sculptured.

278. Formation of the Egg.-The egg originates within peculiar organs, called ovaries, which are glandular bodies, usually situated in the abdominal cavity. So long as the eggs remain in the ovary, they are very minute in size. In this condition they are called ovarian, or primitive eggs. They are identical in all animals, being, in fact, merely little cells (v) containing yolk, (y,) and including other smaller cells, the germinative vesicle, (g,) and the germinative dot, (d.) The yolk itself, with its membrane, (v,) is formed while the egg remains in the ovary. It is afterwards enclosed in another envelope, the shell membrane, which may remain soft, (s,)

Fig. 98.

S

or be further surrounded by calcareous deposits, the shell proper, (Fig. 101, s.) The number of these eggs is large, in proportion as the animal stands lower in the class to which it belongs. The ovary of a herring contains more than 25,000 eggs; while that of birds contains a much smaller number perhaps one or two hundred only.

279. Ovulation. Having attained a certain degree of maturity, which varies in different classes, the eggs leave the ovary. This is called ovulation, and must not le confounded with the laying of the eggs, which is the subsequent expulsion of them from the abdominal cavity, either immediately, or through a special canal, the oviduct. Ovulation. takes place at certain seasons of the year, and never before the animal has reached a particular age, which is commonly that of its full growth. In a majority of species, ovulation is repeated for a number of years consecutively, generally in the spring in terrestrial animals, and frequently several times a year; most of the lower aquatic animals, however, lay their eggs in the fall, or during winter. In others, on the contrary, it occurs but once during life, at the period of maturity, and the animal soon afterwards dies. Thus the butterfly and most insects die, shortly after having laid their eggs.

280. The period of ovulation is one of no less interest tc the zoologist than to the physiologist, since the peculiar characteristics of each species are then most clearly marked. Ovulation is to animals what flowering is to plants; and, indeed, few phenomena are more interesting to the student of nature than those exhibited by animals at the pairing season. Then their physiognomy is the most animated, their song the most melodious, and their attire the most brilliant. Some birds appear so different at this time, that zoologists are always careful to indicate whether or not a bird is represented at the breeding season. Fishes, and many other animals, are ornamented with much brighter colors at this period