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And now we have a perfect cell-structure, differing from an ordinary cell only in having the inner sac, inclosing the dot, on the side, instead of in the centre. The outer membrane corresponds to the Ectoblast, or outer cell sac, the Purkinjean vesicle to the Mesoblast, or inner cell sac, while the dot in the centre answers to the Entoblast.
At this time we are again reminded of Saturn and its rings, which seems to have its counterpart here. These rings disappear, and now once more out of the yolk mass loom up little dots as minute as before; but they are round instead of angular, and those nearest the Purkinjean vesicle are smaller and clearer, containing less of oil than the larger and darker ones on the opposite side.
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.
This is produced by a concentration of the albumen, which now separates from the oil and collects at the upper side of the egg, forming this light spot, called by naturalists the Purkinjean vesicle, after its discoverer, Purkinje. When this albuminous spot becomes somewhat larger, there arises a little dot in the centre, the germinal dot, as it is called.
They all assume the ordinary cell character, with outer and inner sac, the inner sac forming on the side, like the Purkinjean vesicle itself; but it does not retain this position, for, as soon as its wall is formed and it becomes a distinct body, it floats away from the side and takes its place in the centre.
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.
Presently the whole becomes redissolved: these stages of the growth being completed, this little system of worlds is melted, as it were: but while it undergoes this process, the albuminous spheres, after being dissolved, arrange themselves in concentric rings, alternating with rings of granules, around the Purkinjean vesicle.
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