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With all, as far as is at present known, the germinal vesicle is the same; so that all organisms start from a common origin. If we look even to the two main divisions namely, to the animal and vegetable kingdoms certain low forms are so far intermediate in character that naturalists have disputed to which kingdom they should be referred.
This egg consists of an outer envelope, the vitelline membrane, containing a fluid more or less dense, the yolk; within this is a second envelope, the so-called germinative vesicle, containing a somewhat different and more transparent fluid, and in the fluid of this second envelope float one or more so-called germinative specks.
While the lenticular sac is being detached and is causing the invagination of the primary optic vesicle, another invagination is taking place from below; this proceeds from the superficial part of the skin-fibre layer the corium of the head. In this way the optic vesicle acquires the form of a hood. It originates from the part of the head-plates which immediately encloses the eye.
The first change in the yolk, after the formation of the Purkinjean vesicle, is the appearance of minute dots near the wall at the side opposite the vesicle. These increase in number and size, but remain always on that half of the yolk, leaving the other half of the globe clear.
It needs to be continuously first built up by food, and then broken down by discharging what is no longer needful for its healthy existence. Thus the life of every organism is a life of almost incessant change, not only in its being as a whole, but in that of all its protoplasmic particles also. The clear space within it is a contractile vesicle. The dark body is the nucleus.
The mechanical difficulties opposing direct nutrition through the placenta, and the impossibility of nourishment by this method during the early stages of embryonic life previous to the formation of the placenta or umbilical vesicle. "7th.
However, in the lamprey the spinal cord swells in front into a simple pear-shaped cerebral vesicle, and at each side of it there are a very simple eye and a rudimentary auditory vesicle. The nose is a single pit, as in the Amphioxus. The two sections of the gut are also just the same and very rudimentary in the lamprey.
But in some of the pale-blooded and colder animals, as in snails, whelks, shrimps, and shell-fish, there is a part which pulsates, a kind of vesicle or auricle without a heart, slowly, indeed, and not to be perceived except in the warmer season of the year.
Its effects fall as far short of what might have been expected from its virulence as the pearly vaccine vesicle falls short of the terrors of the confluent small-pox. They might with as much propriety call them matricides, if they did not agree with the milder teachings of their mothers. I can imagine Jonathan Edwards in the nursery with his three-year-old child upon his knee.
Only one spermatozoon can bore its way into the yelk at one pole of the ovum-axis; its head or nucleus coalesces with the female nucleus, which remains after the extrusion of the directive bodies from the germinal vesicle. This now undergoes total segmentation, dividing into two, four, eight, sixteen, thirty-two cells, and so on.
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