Oviparity

“ CHAPTER III. ELASMOBRANCHII. The impregnation of the ovum is effected in the oviduct. In most forms the whole of the subsequent development, till the time when the embryo is capable of leading a free existence, takes place in the uterus; but in other cases the egg becomes enveloped, during its passage down the oviduct, first in a layer of fluid albumen, and finally in a dense horny layer, which usually takes the form of a quadrilateral capsule with characters varying according to the species. ”

“ The queen, it will be remembered, cohabits with the male but once in her life. The sperm-cells then received are stored in a special receptacle and are released during the passage of the egg down the oviduct. In this act of releasing the fertilizing germs a singular economy is practised. In the case of most other creatures myriads of sperm-cells are released for the fertilization of a single egg, and of these but one can possibly attain its goal, the minute aperture or “micropyle” which is the doorway to the germ liberated, in the form of an egg, by the female. The rest die. ”

“ The uncertainty becomes still greater owing to our inadequate knowledge as to the length of time during which the sexual elements, the ova and the spermatozoa, retain their vitality after liberation from their respective sources. While it is not certainly known, it is probable that the ovum is capable of impregnation any time during its sojourn within the oviduct and before reaching the uterus, or probably for a period of about one week from the time of its escape from the Graafian follicle. ”

“ The difference is this: true parthenogenesis takes place in perfect females, capable of sexual union and of fertilization, possessing perfect ovaries and producing true ovules which develop into embryos without fertilization. The larval aphid, on the contrary, is not a perfect female, is not capable of sexual union nor of fertilization; its ovary-like organ is not a true ovary, does not produce true ovules which develop into embryos, but forms an embryo at once within, which then is born in an active state. ”

“ And thus there is formed, not a new part, but a new whole; it is not an organ which is kept up, but an offspring which is prepared. The progeny is 217 included in the parent, and is gradually fitted to be separated from it. The young is at first only the development of a part of the organization of the mother;—of a germ, an ovule. But it is not developed like other organs, retaining its general form. It does not become merely a larger bud, a larger ovule; it is entirely changed; it becomes—from a bud—a blossom, a flower, a fruit, a seed; from an ovule it becomes an egg, a chick, a bird ”

“ As the bird approaches maturity, one ovary and its oviduct enlarge, and the ova, which develop from the inside of the ovary just as the ovule develops inside the flower ovary, also become large. Although the bird is born with two ovaries, but one, usually, develops, generally the one on the left side.
When the bird comes to maturity, there is born in it a yearning for home and offspring. As the eggs develop, the bird turns to the nest and to the mate who is to share with her all this beautiful life. ”

“ The site being mutually agreed upon, the happy pair proceed with all possible dispatch and diligence to construct a domicile: the male to collect and bring in the necessary materials; the female to fix them in their proper places.... Having finished their home, only a day or so intervenes when oviposition becomes the controlling instinct. The female now proceeds to deposit her complement of four eggs, which she does on consecutive days, at the rate of a single egg daily. This is followed, on the day succeeding the last deposit, by the trying duty of incubation. ”

“ All alike are generated from sexual union, the male emitting semen into the female. But among the ovipara (1) birds produce a perfect hard-shelled egg, unless it be injured by disease, and the eggs of birds are all two-coloured. (2) The cartilaginous fishes, as has been often said already, are oviparous internally but produce the young alive, the egg changing previously from one part of the uterus to another; and their egg is soft-shelled and of one colour. One of this class alone does not produce the young from the egg within itself, the so-called ‘frog’ ”

“ Sexual reproduction is quite a distinct matter. Here, in all these cases, what is required is the detachment of two portions of the parental organisms, which portions we know as the egg or the spermatozoon. In plants it is the ovule [395] and the pollen-grain, as in the flowering plants, or the ovule and the antherozooid, as in the flowerless. Among all forms of animal life, the spermatozoa proceed from the male sex, and the egg is the product of the female. ”

“ This true ovary is somewhat lateral in most Rotifers, but is median in Asplanchnidae, and paired in Pterodina, Bdelloida, and Seisonaceae. A membranous covering is common to the ovary and yolk-gland (paired when these are paired) ; it is continued into a thin-walled tube or oviduct, which opens into the cloaca on its ventral side beyond the bladder or common renal duct. In the viviparous species the mature ovum (Fig. 112, em) usually lies in the oviduct, dilating it into a sort of "uterus" until the birth of the young. ”

“ Reproduction.—Do you see any swellings upon the side of the hydra? (Fig. 34, A.) If the swelling is near the tentacles, it is a spermary; if near the base, it is an ovary. A sperm coalesces with or fertilizes the ovum after the ovum is exposed by the breaking of the ovary wall. Sometimes the sperm from one hydra unites with the ovum of another hydra. This is called cross-fertilization. The same term is applied to the process in plants when the male element, developed in the pollen of the flower, unites with the female element of the ovule of the flower on another plant. ”

“ The transition from this system of reproduction to that by which the fœtus is formed into a greater or less degree of development within the body of the parent, and then brought forth to be nourished into further development by the parental milk, is enormous. The principle of the organic construction and mode of perpetuating the species, in the two cases, is absolutely unlike after we pass the point at which the ovule is formed by the union of the male and the female vesicles that are supposed to constitute its substance. ”

“ The male, however, is much smaller, and far less developed than the female. In some of these species, germ-cells, or eggs, are found, which do not require fecundation for reproduction or development, so that they belong to the non-sexual class.
The third variety of hermaphrodites embraces those animals in which the male organs are so disposed as not to fecundate [pg 201] the ova of the same body, but require the co-operation of two individuals, notwithstanding the co-existence in each of the organs of both sexes. Each in turn impregnates the other. ”

“ The reproductive organs.—The internal reproductive organs are situated in the abdomen; there is a set on each side, but the two sets open by a common duct, whose outlet is at the hind end of the body.
The reproductive organs of the female are shown on Plate XXVI (a) , Fig. 1. There are two ovaries (o) , one on each side of the body. Each ovary consists of a large number of parallel egg-tubes, within which the eggs are developed. The egg-tubes of each ovary open into an oviduct (od) . The two oviducts unite and form a single tube on the middle line of the body ”

“ Posteriorly they end in two short oviducts which open into a uterus, in which fertilisation takes place, and in which the secretion arises which cements the eggs together. In the adult the ovaries and a receptaculum seminis are found, in addition to the organs present in the larva. The ovaries are formed from modifications of the packing tissue; they begin close behind the head, and soon attain such dimensions as to compress the egg-sacs and body-cavity to small slits. ”

“ Ovipositor.—If the specimen is a female, it has an egg-placer or ovipositor, consisting of four blunt projections at the end of the abdomen (Fig. 107) . If it is a male, there are two appendages above the end of the abdomen, and smaller than the parts of the ovipositor. Females are larger and more abundant than males. In laying the eggs, the four blunt points are brought tightly together and then forced into the ground and opened (Fig. 108) . By repeating this, the grasshopper makes a pit almost as deep as the abdomen is long. The eggs are laid in the bottom of the pit. ”

“ The ovaries always contain spermatozoa, some of which project through the ovarian wall into the body cavity. Spermatozoa are not found in the uterus and oviducts, and it appears probable that they reach the ovary directly by boring through the skin and traversing the body cavity. [6] In the neotropical species there is a globular receptaculum seminis opening by two short ducts close together into the oviduct, and there is a small receptaculum ovorum with extremely thin walls opening into the oviduct by a short duct just in front of the receptaculum seminis. ”

“ The elaboration of both ovules and sperm in the same individual is wasteful of vital energy. The concentration of vital energy on one reproductive element secures that one product in a higher degree of perfection. Thus better sperm and better ovules combine to produce better ova and a stronger embryo. This is in accordance with the effect of differentiation of functions and organs of all kinds. 2. Again, in all cross-fertilization different individual characteristics are inherited by the common offspring. ”

“ In short, that many of the ovaries are now normally abortive, though Nature occasionally makes futile efforts for their perfect development, in the production of ovuloid bodies; these then I regard as the male flowers. The others that are still capable of fertilisation, and likewise possessing male organs, are hermaphrodite, and must, I think, from the results of your comparative examinations, present a somewhat different condition; as it can scarcely be supposed that ovules in the condition you describe could ever be fertilised. ”

“ There is therefore no room for hesitation, strange though the statement may appear: the egg, as it descends from its ovarian tube, has no determined sex. It is perhaps during the few hours of its rapid development at the base of its ovarian sheath, it is perhaps on its passage through the oviduct that it receives, at the mother's pleasure, the final impress that will produce, to match the cradle which it has to fill, either a female or a male. ”

“ Animals, when capable of sexual reproduction, are fully developed, and it is scarcely possible that the male element should affect the tissues of the mother in the same direct manner as with plants; nevertheless it is certain that her ovaria are sometimes affected by a previous impregnation, so that the ovules subsequently fertilised by a distinct male are plainly influenced in character; and this, as in the case of foreign pollen, is intelligible through the diffusion, retention, and action of the gemmules included within the spermatozoa of the previous male. ”

“ In connexion with the structure of the ova, the ovaries themselves and the oviducts are built upon the Sauropsidan plan. In the male the testes retain the primitive abdominal position. The fact that the urinary and genital products escape by means of their ducts into a chamber which also receives the end of the alimentary tract is not a distinctive feature of this group, inasmuch as it is seen in the Marsupials, and also in certain low Eutheria, such as the Beaver and other Rodents, and a few Insectivores. ”

“ Physicians have repeatedly propounded the theory that one ovary produces eggs capable of developing only into females, the other only those capable of becoming males, and the suggestion has been made that in the case of human beings ovulation takes place alternately from the ovaries. From this it would follow that were the sex resulting from one fertilization known, the sex of a subsequent fertilization could be predicted, or by choosing the date of fertilization, selected. ”

“ During embryological development the fertilized ovum—consisting of two fused fragments of this germinal substance—gives rise to a host of ordered and marshalled cells, which are divisible into two groups: the one forms the body with its muscles, bones, glands, digestive system, skin, sense-organs, nerve-centres, and so forth; the other forms a reserve store of germinal substance, from which are derived the ova and sperms. The former take no direct share in reproduction; they are off the line of continuous descent; they die without issue. ”

“ These are closely set upon the inner surface of the oviduct; their outer ends are free and continually sway to and fro like a wheat field on a windy day; and by their motion they create a current in the direction in which the ovum should move, namely, toward the uterus. While passing through the oviduct, the ovum has no attachment whatever to the mother, yet development is going on all the time. It is thus made perfectly clear that development is not directed by the parent. ”

“ But whatever difference takes place in generation, Nature, by a new production, prepares the body for it, and which, whether manifested outwardly, or concealed internally, always precedes generation. The ovaries of oviparous animals, and the testicles of female viviparous animals, before the season of impregnation, experience a considerable change. Oviparous animals produce eggs, which at first are attached to the ovaries, by degrees [216] they increase in size, until they fall into the canal of the matrix, where they acquire their white membranes, and shell. ”