In general, the new nuclei in the cells formed by division are not made de novo, but arise from the splitting of the nucleus in the mother-cell. The new nucleus assimilates material, grows to maturity, and divides again into two daughter-nuclei. Whatever be the number of chromosomes that enter a new nucleus as it forms, the same number issues from it in mitosis. Boveri said,[31] "We may identify every chromatic element arising from a resting nucleus with a definite element that enters into the formation of that nucleus, from which the remarkable conclusion follows that in all cells derived in the regular course of division from the fertilized egg, one half of the chromosomes are of strictly paternal origin, the other half of maternal." It is not strictly true to say that the germ-nuclei fuse: they send in two sets of chromosomes that lie side by side, as has been frequently demonstrated since 1892[32] in many of the lower forms of life, and this law almost certainly extends also to man.
The primordial germ-cells appear in the human fetus about the twentieth day and finally mature at puberty. Then an ovum at menstruation breaks out through the surface of the ovary, and is taken by the fimbriae of the Fallopian tube into the lumen of this tube. Fecundation happens near the outer or ovarian end of the Fallopian tube, and the fecundated ovum finally is passed on to fasten on the wall of the uterus. The spermatozoön is a ciliated cell with the power of locomotion, through the movement of the tail of the cell. It can move 0.05 to 0.06 mm., or its own length, in a second. It thus passes up through the uterus and out through the Fallopian tube, against the cilary motion of the tubal cells, until it meets the ovum.
A human ovum is a typical cell, but it has a covering membrane, and a minute quantity of deutoplasm or yolk, which is not alive, and is food for the growing embryo before the embryo begins to draw sustenance through the placenta. The eggs of birds have a large quantity of food stored in the yolk, since their embryos live in the ovum and draw food therefrom during the entire period which corresponds to the time of gestation in mammals. The "white" and the calcareous shell of a hen's egg are adventitious parts, added in the oviduct after the egg leaves the ovary.
The spermatozoön is a complicated organism. The head is partly covered with a thin protoplasmic cap, and it contains the nucleus with the chromatin. In the neck are two centrosomes. The tail is in three parts with an axial filament throughout, which is a bundle of extremely minute fibrils. In the middle part the axial filament is surrounded by an inner sheath; outside this sheath is a spiral filament lying in a clear substance; and outside the spiral filament is a finely granular layer of protoplasm, called the Mitochondria. This organism is a living animal cell, and it can live in an incubator, or in the Fallopian tube for two or three weeks, altogether removed from the living male body that produced it. Sir John Lubbock[33] says he kept a queen ant alive for thirteen years. This ant, which died in 1888, had been fertilized in 1874, and never afterward. She laid fertile eggs for thirteen years; that is, the spermatozoa in her oviduct retained their vitality for thirteen years.
The human spermatozoön is a living cell: it has (1) the requisite structure; (2) the chemical composition of an organic being; (3) a figure in keeping with its species; (4) an origin from a living progenitor; (5) the explicatio naturae; (6) the power of assimilation; (7) the duratio viventium; (8) the power of reproduction; (9) motion and locomotion. As soon as the ovum breaks through the surface of the ovary it has all the qualities of the spermatozoön except locomotion. These two cells are animal cells, not vegetable; just as single-celled protozoa, like Actinophrys, Actinosphaerium, Closterium, Stentor, and the Amoebas are animals, not plants. It is not possible in our present knowledge sharply to differentiate ultimate forms of plants from animals. To say that animals have the qualities of plants plus a sentient vital principle is not enough. It is very doubtful that even the so-called sensitive plants feel, and it is practically certain that many low forms of animal life do not feel—they have no sentient mechanism. Plants have the qualities enumerated above plus the power of drawing nutriment directly from inorganic material, while animals can draw nutriment directly only from organic material; yet some fungi, bacteria for example, will grow and thrive only on organic material, and animals will take up mineral drugs. It is questionable, however, that minerals which thus find a way into animal cells are really assimilated. They excite or irritate these cells into intenser action, and thus cause growth, rather than affect development by direction. The so-called mineral tonics used in medicine act by irritation.
This irritation or stimulation by drugs can in certain very low forms of animal life start mitosis in the unfertilized ovum, and thus build up part, at the least, of a specific embryo parthenogenetically: here probably a polar body takes the place of the spermatozoön. Loeb, by treating the unfertilized egg of Arbacia (a sea-urchin) with magnesium chloride, started mitosis that resulted, it is said, in a perfect Pluteus larva.[34]
The human ovum is about half the size of a period in the type of this page, and two hundred and fifty spermatozoa will fit side by side along the horizontal diameter of the lowercase letter o here. The nuclei of these cells are extremely minute: they must be stained and be observed with a high-power objective on the microscope before they become visible. This small nucleus of the spermatozoön penetrates the covering membrane of the ovum, enlarges, and becomes the male pronucleus. The pronucleus unites permanently with the pronucleus of the ovum, and together they form the Cleavage or Segmentation Nucleus of the fertilized ovum. This new nucleus gives rise by division to the innumerable myriads of nuclei in the growing body. Hence every nucleus of the child apparently contains nuclear material derived from both parents, as has been said.
The two perfected germ-cells before fecundation are in a state of nuclear rest after the numerous mitotic changes that have taken place in the maturation of these cells. When these nuclei unite in the ovum an intense activity at once is set up. Biologists offer very many theories to explain this awakening force. Herbert Spencer, Herting, and others held that protoplasm when perfected tends to pass into a state of stable equilibrium and consequent lessened activity, but fertilization restores it to a labile state. This and similar theories are verbose amplifications of the obvious fact that the cells start to divide and the biologists do not know the cause. The soul, of course, cannot have anything to do with the matter, because you cannot smell a soul. "Senescence and rejuvenescence" is another sonorous explanation that does not explain, used by Minot, Engelmann, and Hansen. Weismann rejects these theories for his own "Fertilization as a Source of Variation." Anyhow, the fertilized cell starts to divide regardless of the biologists. Adult cells may be stimulated to divide by chemical irritation, by mechanical pressure as in the formation of calluses, traumatism, by any agency that brings about an abnormal condition of the body, but this fact does not explain the normal fission of the fecundated ovum.
In about fifteen days from the date of fertilization the ovum passes through the following stages:
1. The ovum, with a full series of mitotic changes of the ordinary somatic type described above, divides, subdivides, and grows within the cell-wall until a rounded mass of cells is formed, which is called the Morula or Blastula—the original cell-wall, of course, stretches to hold these new cells. They are of unequal size, and they divide at unequal rates.