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The quality of maleness consists only in the size, form, and mobility of the sperm in the higher animals and of the microgamete in other cases. In what sense then, can an ovum be male?
These two structures, which from now on constitute the ovum, can best be considered separately and in the order of their development. We shall therefore first study the sac and in the next chapter the embryo.
It is supposed that the ovum is grasped by the fringe-like extremity of the Fallopian tube and is carried through it by the movements of the ciliary epithelium to the uterus. The formation of new follicles continues only for a short time after birth, when the Graafian follicles are the most numerous; the entire number contained within the ovaries of the child being estimated at over 70,000.
As a final result of these two factors, the maternal tissue which covers the ovum becomes thinned and stretched; it is pushed entirely across the uterine cavity; and by about the twentieth week meets the opposite side of the cavity, to which it becomes adherent.
Connective tissue. In all higher animals and plants the life of the individual begins as a single ovum or a single cell, and as it grows the cells increase rapidly until the adult is formed out of hundreds of millions of cells. As these cells become numerous they cease, after a little, to be alike. They assume different shapes which are adapted to the different duties they are to perform.
The individual man needs only nine months for his complete development, from the fecundation of the ovum to the moment when he leaves the maternal womb. In many other mammals the time of the embryonic development is much the same as in man for instance, in the cow. In the horse and ass it takes a little longer, forty-three to forty-five weeks; in the camel, thirteen months.
But neither of these functions can be performed without the participation of the uterine mucous membrane, the soil, as it were, in which the ovum is planted. We must now learn how the maternal tissues assume the responsibility placed upon them.
This tiny, living sphere, it will be recalled, reaches the womb a few days after conception, and adheres to the uterine mucous membrane. At first, however, its roots are short and delicate, and not so capable of anchoring the ovum as they become later. It is only toward the end of the eighteenth week that the union between the womb and its contents becomes firm.
Although the microscope made impossible this very simple explanation, it gave in return a truer, if more complex, account of the transformation from egg to offspring. By this means it has been definitely proved that the ovum multiplies rapidly after it has been fertilized, and becomes, as was explained in the preceding chapter, a sac-like structure within which hangs a tiny clump of tissue.
In the economy of the reproduction of the species of animals, one of the most important circumstances is the aeration of the ovum, and when this is not performed, from the blood of the mother as in the mammalia by the placenta, there is a system for aerating as in the oviparous reptiles or fishes, which enables the air freely to pass through the receptacles in which the eggs are deposited, or the egg itself is aerated out of the body through its coats or shell, and when air is excluded, incubation or artificial heat has no effect.
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