The eggs of all mammals look alike as they come from the ovaries, but take on some changes afterward. Hæckel says: "Every primitive egg being an entirely simple, somewhat round, moving, naked cell, possesses no membrane, and consists only of a nucleus and protoplasm. These two parts have long borne distinctive names: the protoplasm being called the vitellus, or yelk, and the nucleus the germinal vesicle (vesicula germinativa)." The same author also says: "The human egg cannot be distinguished from that of most other mammals, either in its immature or in its more complete condition. Its form, its size, its composition, are approximately the same in all. In its fully developed condition it has an average diameter of one-tenth of a line—about the one hundred and twentieth part of an inch. If the mammalian egg is properly isolated, and held on a plate of glass towards the light, it appears to the eye as a very fine point. The normal eggs of most of the higher mammals are of almost exactly the same size. They have the same spherical form; always the same characteristic covering; always the same clear, round germinal vesicle with its dark germinal spot. Even under the highest power of our best microscopes there appears to be no essential difference between the eggs of a human being and that of the ape, the dog, the cat or other animal." This similarity is one of appearance only. There is a difference, and of this I shall speak later. It may be asked if the egg of a bird is the same as the egg of a mammal. The mature bird's egg, as it is laid in the nest, differs materially from that of any mammal; but in its miniature form, as found in the hen's ovary, it is also the same. The egg of a bird after it leaves the ovary, and as it passes along the oviduct, takes on secretions in its passage which it converts into yelk, and afterwards a shell is added to give it protection in the external world, where it must undergo incubation before it can become a bird; but before it takes on its shell it has been fertilized, and this also causes other changes. Hæckel says: "After the ripe egg of the bird has left the ovary, and has been fertilized in the oviduct, it surrounds itself with various coverings which are secreted from the inner surface of the oviduct. The thick layer of transparent albumen first forms round the yellow yelk; this is followed by the formation of the outer calcareous shell, within which is another envelope, or skin. All these coverings and additions which are gradually formed round the egg are of no importance to the development of the embryo; they are parts which have nothing to do with the simple egg cell. Even in the case of other animals we often find large eggs with thick coverings. For example, the shark's; but even in this case the egg is originally exactly similar to those of mammals when in its primitive condition as it comes from the ovary. In the case of the bird these additions serve only as food for the growing embryo, which, in the case of mammals, is furnished by a stream of the mother's blood, making 'stored-up' nutriment unnecessary."
Before, however, we can have true germ plasm the mother cell must be fertilized by the male cell. This is true of all the higher plants and animals. There are some low plants and animals in which fertilization by the male cell is not required. This has been called virginal generation. In no mammal is this possible.
How fertilization takes place and what it signifies are both important questions which have not been entirely settled, and it almost seems as if they could not be settled in some of their details, except in the lower forms of life. Nature has so protected the process from observation in the higher animals that it cannot be studied in detail; but in plants and the lowest animals it has been observed with some success, and we may infer that the process is very much the same in the higher animals.
Hæckel, in his great work on the Evolution of Man, tells us that "The process of fertilization in sexual generation depends essentially on the fact that two dissimilar cells meet and blend. In former times the strangest views prevailed with regard to this act. Men have always been disposed to regard it as thoroughly mystical, and the most widely different hypotheses have been framed to account for it. It is only within a few years that closer study has shown that the whole process of fertilization is extremely simple, and entirely without special mystery. Essentially, it consists merely in the fact that the male sperm-cell coalesces with the female egg-cell. Owing to its sinuous movements, the very mobile sperm-cell finds its way to the female egg-cell, penetrates the membrane of the latter by a perforating motion, and coalesces with its cell material.
"A poet might find in this circumstance a capital opportunity for painting in glowing colors the wonderful mystery of fertilization; he might describe the struggles of the 'seed animalcules' eagerly dancing round the egg-cell shut up in its many coverings, disputing the passage through the minute pore-canals of the chorion, and then of purpose burying themselves in the protoplasm of the yelk mass, where, in a spirit of self-sacrifice, they completely efface themselves in the better 'ego.' But the critical naturalist very prosaically conceives this poetical incident, this 'crown of love,' as the mere coalescence of two cells! The result of this is, that in the first place the egg-cell is rendered capable of further evolution, and, secondly, that the hereditary qualities of both parents can be transmitted to the child."
By coalescence is understood, growing together, not mingling as water and milk might when mixed. More recent observations indicate that during coalescence both the male and female cells throw off some portions of their substance. It is also considered that the important part of each cell is its nucleus. In it all hereditary characteristics are stored up. If the nucleus be absent in either cell these cells cannot reproduce. In unicellular, or one-celled, organisms, it has been found in multiplication by division, a part of the nucleus must go with each half, otherwise the half without a part of it does not grow. In experiments in laboratories, artificial division of simple organisms may be made, and each fragment will become a perfect creature if only a very small piece of the nucleus goes with the separated portion; but if a part is cut off without any of the nucleus, then, while it may live on for a short time, it can not grow or propagate.
Possibly we have here an explanation of some hereditary phenomena in human beings. If there is an unequal division, and more of the male than of the female nucleus, the child might, as a result, inherit more of the father's than of the mother's characteristics, or the reverse.
What has been so far said about the germ plasm has been to enable the reader to possess a degree of intelligence on the nature of fertilization, so far as it is known; but from a practical standpoint the most important knowledge for those prospective parents who wish to practice intelligent stirpiculture is to understand that the health of the germ plasm or fertilized ovum depends on the health of the parents. By health, I mean the possession of a good constitution, to which will be added a strong hold on life, power to do and to endure, and quickly to recover from weariness. Disease will be easily warded off in such persons, so that there will be generally good health. Such a condition of body is usually inherited. It depends on the possession of a large supply in the body of living matter—firm muscles, a good heart, lungs and digestive organs. Those who are feeble cannot endure much; whose heart, lungs and digestive organs are weak; whose hold on life is slight, can rarely endow their offspring with these high qualities. Their children may live if no great strain comes upon them; but if they must take an active part in the struggle and competition going on in the world they cannot endure it. Mr. Spencer puts the case very aptly in his work on Ethics where he says: "It results that where maternal vigor is great, and the surplus vitality consequently large, a long series of children may be borne before any deterioration in their quality becomes marked; while, on the other hand, a mother with but a small surplus may soon cease altogether to reproduce. Further, it results that variations in the state of health of parents which involves variations in the surplus vitality have their effects on the constitutions of offspring to the extent that offspring borne during greatly deranged maternal health are decidedly feebler. And then, lastly and chiefly, it results that after the constitutional vigor has culminated, and there has commenced that gradual decline which in some twenty years or so brings absolute infertility, there goes on a gradual decrease in that surplus vitality on which the production of offspring depends, and a consequent deterioration in the quality of such offspring. This which is a priori conclusion is verified a posteriori.
"Mr. J. Mathews Duncan, in his work on Fecundity, Fertility, Sterility and allied topics, has given results of statistics which show that mothers of twenty-five bear the finest infants, and that from mothers whose ages at marriage range from twenty to twenty-five years there come infants which have a lower rate of mortality than those resulting from marriages consummated when the mothers' ages are smaller or greater. The apparent slight incongruity between these two statements being due to the fact that whereas marriages commenced before twenty and twenty-five cover the whole of the period of highest vigor, marriages commenced at five and twenty cover a period which lacks the years during which vigor is rising to its climax and includes only the years of decline from the climax."