United States or South Sudan ? Vote for the TOP Country of the Week !
Although every ovum that will be produced as long as a woman lives has already sprung into existence by the time she is born, not a single one ripens for from twelve to fifteen years. The ripening process begins about the time of puberty, and, unless suspended through the occurrence of pregnancy, continues until the menopause.
At the lower or vegetal pole of the ovum a crown of eight large entodermic cells remains for a long time unchanged, while the other cells divide, owing to the formation of a series of horizontal circles, into an increasing number of crowns of sixteen cells each. This layer is the blastoderm, the simple epithelium from the cells of which all the tissues of the body proceed.
Palingenesie Philosophique, part X. chap. But, thus defined, the germ is neither more nor less than the "particula genitalis" of Aristotle, or the "primordium vegetale" or "ovum" of Harvey; and the "evolution" of such a germ would not be distinguishable from "epigenesis."
Let us inquire regarding the function of this alkaline albuminous secretion from the vesicles and prostate. For what purpose does Nature prepare such a secretion? The spermatozoa frequently remain several days in the organs of the female before the ovum is found and fertilized. During these several days the spermatozoa are exerting no small amount of energy in their vigorous flagellate movement.
This activity is present almost from the first a few days after the impregnation of the ovum in the breasts, and quickly becomes obvious to inspection and palpation. Before a quite passive organ, the breast now rapidly increases in activity of circulation and in size, while certain characteristic changes begin to take place around the nipples.
In previous discussions of the subject I have urged that the only meaning of sex was the difference between the megagamete or ovum, and the microgamete or sperm.
On the other hand, single ovum, or identical, twins are always of the same sex; this follows, since but one egg and but one spermatozoon are here concerned. The incident permitting twins to develop from a solitary ovum must occur soon after conception has taken place. It will be remembered that the first step in the development of the fertilized ovum consists in its dividing into two cells.
Provided the requisite conditions for such development are met, these cells are capable of developing into human beings. Each of these remarkable units is called an Ovum, or egg-cell, and represents one variety of the germinal cells. But the other variety, represented by the Spermatozoon and developed only in the male sex, is also required for the production of a human being.
For a time after this sac, or ball, as you may choose to think of it, becomes implanted in the uterus, every part of its wall shares in the responsibility of procuring nourishment for the embryo. On this account the wall, or capsule, is for several weeks the most conspicuous part of the ovum.
Formerly we regarded these congenital or innate characters as derived from the parents or inherited, and heredity was the transmission of constitutional characters from parent to offspring. Now that we fix our attention on the fertilised ovum or the gametes by which it is formed we see that the characters are determined by some properties in the constitution of the gametes.
Word Of The Day