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One of these alterations will be more easily understood if we still think of the ovum as a seed, for it grows away from its roots just as plants do. Most of the capsule, therefore, is removed step by step farther from its source of nourishment, for the maternal blood-vessels do not follow the expanding sac but retain their original position at its base.
While these changes are taking place the growing ovum is carried down the oviduct a distance of four to six inches and finally comes to rest in the uterus, where it is to dwell during the months necessary to its complete development.
For, if it should happen that the first coitus should take place only a day or two before the time when another "monthly" was due, such excitement might hasten the passage of the nearly-ripe ovum into the uterus, and conception might occur. In which case, "all the fat would be in the fire," nothing would be proved, and the parties would be as ignorant as ever regarding the facts in their case.
In connection with menstruation we noted that this membrane periodically prepares for the reception of an ovum. And if the expected ovum has been fertilized, its arrival is followed by arrangements for its protection and nutrition which are far more elaborate than the preparations for its reception.
The reproductive cells also undergo division and increase in number, and when they separate from the new individual and unite in fertilisation they still possess all the determinants of the fertilised ovum from which they are descended. Heredity thus continues from gamete to gamete, not from zygote to soma, and then from soma to gamete.
The generative idea resembles those concepts which, in the sciences, are of wide range because they condense a generalization rich in consequences. The subject is at first comprehended as a whole; development is organic, and we may compare it to the embryological process that causes a living being to arise from the fertilized ovum, analogous to an immanent logic.
The whole of that surface has been made ready to receive it; yet the area actually required to imbed the tiny object is extremely small. As the ovum escapes from the oviduct and enters the womb, it is smaller, in all probability, than the head of a pin. For at least a week after its coming, diligent search is necessary to find the site of implantation.
They are only 1/250 of an inch in diameter, and thus are only half the size of the mammal ova, and have no distinctive features. Fecundation takes place when these lively ciliated cells of the sperm approach the ovum, and seek to penetrate into the yelk-matter or the cellular substance of the ovum with their head-part the thicker part of the cell that encloses the nucleus.
During the second day the Amphioxus-embryo undergoes few other changes. The number of primitive segments increases, and generally amounts to fourteen, some forty-eight to fifty hours after impregnation. It now begins to nourish itself independently, as the food material stored up in the ovum is completely used up.
It is biotic energy which guides the development of the ovum, which regulates the exchanges of the cell, and causes such phenomena as nerve impulse, muscular contraction, and gland secretion, and it is a form of energy which arises in colloidal structures, just as magnetism appears in iron, or radio-activity in uranium or radium, and in its manifestations it undergoes exchanges with other forms of energy, in the same manner as these do among one another."
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