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Afterwards, when the outline of the foetus is distinctly indicated and it begins to have greater bodily consistence, the vesicle in question becomes more fleshy and stronger, changes its position, and passes into the auricles, above which the body of the heart begins to sprout, though as yet it apparently performs no office.
The nucleated vesicle, the fundamental form of all organization, we must regard as the meeting-point between the inorganic and the organic the end of the mineral and beginning of the vegetable and animal kingdoms, which thence start in different directions, but in perfect parallelism and analogy.
From this time the yolk begins to take its color, the oily cells assuming a yellow tint, while the albuminous cells near the vesicle become whiter.
When the Purkinjean vesicle has completed its growth, it bursts and disappears; but the mass contained in it remains in the same region, and retains the same character, though no longer inclosed as before. At a later stage of the investigation, we see why the Purkinjean vesicle, or inner sac of the egg, is placed on the side, instead of being at the centre, as in the cell.
What can be more widely contrasted than a newly-born child and the small, semi-transparent spherule constituting the human ovum? The infant is so complex in structure that a cyclopædia is needed to describe its constituent parts. The germinal vesicle is so simple that it may be defined in a line.
Whilst the foetus is yet in the form of a soft worm, or, as is commonly said, in the milk, there is a mere bloody point or pulsating vesicle, a portion apparently of the umbilical vein, dilated at its commencement or base.
A very rudimentary skull is developed at the foremost end of their chorda. The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first. It afterwards divides into five successive cerebral vesicles, like the brain of the Gnathostoma.
This vesicle divides at an early stage into an upper and larger and a lower and smaller section. The edges join together in the middle part of each fold, and separate from the utricle, the two ends remaining in open connection with its cavity. All the Gnathostomes have these three canals like man, whereas among the Cyclostomes the lampreys have only two and the hag-fishes only one.
The simple cerebral vesicle that is formed by the expansion of the head-part of this medullary tube divides by transverse constrictions into five, and these pass through more or less the same stages of construction in the human embryo as in the rest of the mammals. As these are undoubtedly of a common origin, their brain and spinal cord must also have a common origin.
In certain animals belonging to every one of the chief groups into which the Metazoa are divisible, the cells of the cell-aggregate which results from the process of yelk-division, and which is termed a morula, diverge from one another in such a manner as to give rise to a central space, around which they dispose themselves as a coat or envelope; and thus the morula becomes a vesicle filled with fluid, the planula.
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