Devices for the Care of Eggs and Young
With most animals that practice internal fertilization, a part of the oviduct becomes modified into a brood organ, the uterus, for the protection of the developing embryo. This structure is located midway between the upper portion of the oviduct, called in human anatomy the Fallopian tube, which receives the egg from the ovary, and the vagina below. The virgin uterus is completely within the pelvis but during pregnancy it is shifted to a position higher up in the abdomen. Its thick muscular walls are capable of great distension, enlarging over 200 times when accommodating a growing fetus. When unoccupied by young the cavity within the uterus is relatively small and the shrunken inner walls are more or less in contact with each other. The rounded mouth of the uterine cervix, where it meets the vagina, frequently projects somewhat into the vaginal cavity.
Even in frogs the oviduct during the breeding season enlarges at its cloacal end into a “uterus” for the temporary lodgment of the eggs. Viviparous teleosts, as also some elasmobranchs, have a well-developed highly vascularized uterus. Mammals, however, show the greatest differentiation of this organ.
A uterine modification of the oviduct, when found among lower vertebrates, is usually a double structure, one for each oviduct, but with increasing coalescence of the oviducts in mammals to form a single vagina, there is a tendency for this fusion to involve either a part or the entire uterine region. All the theoretical intermediary evolutionary stages from a double uterus to a single one have their actual counterparts in nature among mammals.
Thus, there are two distinct uteri (uterus duplex, Fig. 405a) without vaginas in monotremes, and each with a separate vagina in marsupials. Among placental mammals, some rodents, for example the mouse, hare, marmot, and beaver, as well as elephants, certain bats, and the “aard-vark” Orycteropus of South Africa, have a duplex uterus which opens into a single vagina. A beginning of coalescence between the two uteri (uterus bipartitus, Fig. 405b) is apparent in pigs, cattle, certain rodents, some bats, and carnivores. A two-horned uterus (uterus bicornis, Fig. 405c) is characteristic of ungulates, cetaceans, insectivores, and some carnivores, while a single uterus (uterus simplex, Fig. 405d), with two slender Fallopian tubes projecting from it, is the type found in apes and man.
Various pathological anomalies that suggest doubling are encountered in the human uterus, which find a ready explanation in the story of the comparative anatomy of this organ.
The curious South American teleost Girardinus, already mentioned as having a metamorphosed haemal spine of a caudal vertebra for a copulatory organ, has a hollow ovary that serves as a uterus or brood sac for the young. The eggs, which dehisce into the cavity of the ovary after the teleostean tradition, are fertilized in place by sperm that penetrate all the way into the ovarian cavity, where the early stages of embryonic development occur.
Among vertebrates there are various instances, more or less exceptional, of brood sacs for eggs or young, aside from the uterus of the female. For example, among fishes there is a modification of the pelvic fins into a brood sac on the ventral side of the male pipefish, Syngnathus, into which the eggs are deposited by the female. In the sea-horse, Hippocampus, a relative of the pipefish, there is a similar arrangement whereby the male becomes responsible for the care of the eggs.
Among amphibians a dorsal pouch is located on the back of the female frog, Nototrema pygmaeum, of Venezuela, for carrying eggs, while the male Rhinoderma darwini of Java contributes his vocal sacs temporarily to serve as brood pouches.
The transient brood sac of monotremes and the permanent “marsupium” of the marsupials among mammals are further examples of structures belonging to the reproductive apparatus, since they obviously have been developed in the interests of the race rather than of the individual.
Eggs destined to leave the body of the female before development are provided with some sort of a protective envelope or capsule. In water this does not need to be a very complicated structure, but exposure to dry air demands a shell of some kind.
It is apparent that fertilization must occur before the shell is put on to the egg, otherwise the sperm would encounter an insuperable barrier. Consequently nidamental glands, which produce the shell, are located in the walls of the oviduct some distance from the ostium abdominale, in order to allow opportunity for the egg and sperm to meet before the shell is added. Incidentally, putting a workman-like shell around a soft egg is an accomplishment which would be baffling to a human inventor.
Albumen glands, that furnish the “white of the egg,” are also located in the walls of the oviduct between the ostium abdominale and the nidamental glands, since this extra store of nutriment must be added after fertilization, but before the egg is encased in a calcareous shell. The familiar cackling of a hen that has just succeeded in laying an egg is a true song of triumph, stimulating to a comparative anatomist who appreciates something of the intricacies of its elaboration.
Not only does the calcareous shell protect the exposed egg from injury, but, in the case of many birds, blending colors or blotches which help to camouflage it from searching enemies are deposited in the substance of the shell.
The aquatic eggs of the internally fertilized elasmobranchs are enclosed in purselike horny capsules, supplied at each corner with curling tendrils, which entangle them among seaweeds, so that the embryo fishes sway and rock within their curious cradles in comparative security until ready to emerge.
Placenta in Mammals
True mammals provide a placenta for the developing young. This is an elaborate compound vascular organ, made up of interdigitating villi from the walls of the uterus and from the allantois of the embryo (Fig. 406), which brings the capillaries of the mother into intimate contact with the capillaries of the fetus, thus establishing a nutritive and respiratory bridge between mother and offspring.
There are other embryonic devices in mammals, such as the amnion and other fetal envelopes, which provide for the welfare of young reptiles, birds, and mammals. These should not be overlooked in reckoning up the anatomical contrivances that aid in the preservation of the species.