DEVELOPMENT OF THE EN'Illei0. Harvey laid down the principle, in opposition to the views of those who believed in the doctrine of spontaneous generation, that all animals are produced from eggs (mane drum ex ore); and move recent researches have fully confirmed this view, if we arc allowed to accept the modes of reproduction known as gemmiparous and fissiparous, or multiplication by buds and propagation by division, and the series of cases in which the oflspring never resemble their parents, but the original form reappears in the second generation, and which are consequently said to present the phenomena of alternate generation. These exceptional cases occur only in very low forms of animal life, and, as a general rule, in retracing the phases of animal life, we arrive at an epoch in which the incipient animal is inclosed within an egg. It is.then termed an embryo; and the modifications which this embryo undergoes before the young animal has an independent existence, are included in the general term which Stands at the head of this article. Before embryology was properly studied, all animals were arranged under two great heads—the oviparous, which lay eggs; and the viviparous, which bring forth their young alive. We now know that viviparous as well as oviparous animals are produced from eggs—the only difference in this respect being, that their eggs, instead of being laid before the development of the embryo, begin to undergo their early changes in the body of the mother.
The egg has generally a more or less spherical form; the eggs of birds have, how ever, the form of an elongated spheroid, narrow at one end, and hence the origin of the word oval. Some eggs, as, for instance, those of certain insects (e.g., the podurella), are furnished with projecting filaments; others have a sculptured surface; and others, again, have peculiar forms, being cylindrical or prismatic. A simple sphere is, how ever, the normal form.
The egg originates within organs termed ovaries, peculiar to and characteristic of the• female, except in, those cases in which both male and female reproductive organs are associated in the same individual. These ovaries are glandular bodies, and are usually situated in the abdominal cavity. So long as the eggs remain in the ovary, they are very•
minute, and in this condition they are called ovarian or primitive eggs. They are identical in all animals, being, in fact merely little cells, containing yolk substance, in which is inclosed the germinative vesicle and the germinative dot. The yolk itself, with its membrane, is formed while the egg remains in the ovary; it is afterwards inclosed in another envelope, the shell-membrane, which may either remain soft, like parchment, or may be surrounded by calcareous deposit, as we observe in birds' eggs. The number of eggs seems to increase in proportion as we descend the animal scale, the eggs laid by a bird being far fewer than those laid by a fish, while these again are Jess than those laid by some insects.
The egg, when It has attained of maturity, leaves the ovary. This step in the process IS termed' ovulation, and must not be confounded' with the laving or deposition of the eggs, which is their subsequent expulsion from the abdominal cavity, through (in most cases) a special canal termed the oviduct. Ovulation takes place at a definite period of the year in most of the lower animals, and seldom until the animal has attained its full growth. In general, ovulation is repeated for a number of years consecutively, usually in the spring; sometimes, however, it occurs but once during life, as in most insects, which die soon after the process is accomplished.
After leaving time ovary, the eggs are either discharged from the animal, and undergo their further changes in the external world, or they continue their development within the body of the mother, as is the case in some fishes and reptiles (as sharks and vipers), which have consequently been named ovoviviparous animals; or, in the case of the inam inalia, they are not only developed within the body of the mother, but, except in the case of the marsupiata and monotremata (q.v.), become intimately united to her by the intervention of certain temporary structures—namely, the placenta and umbilical cord. This mode of development is termed gestation.