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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.
Together with the lens the small underlying piece of corium-plate also separates from the skin. In both cases the hollowing of the closed vesicle on one side goes so far that at last the inner, folded part touches the outer, not folded part, and the cavity disappears. The hollow stem of the primary optic vesicle is converted into the optic nerve.
The remains of the yelk, which have not yet been applied to the nutrition and growth of the young animal, are contained in a sac attached to the rudimentary intestine, and termed the yelk sac, or 'umbilical vesicle. Two membranous bags, intended to subserve respectively the protection and nutrition of the young creature, have been developed from the skin and from the under and hinder surface of the body; the former, the so-called 'amnion, is a sac filled with fluid, which invests the whole body of the embryo, and plays the part of a sort of water-bed for it; the other, termed the 'allantois, grows out, loaded with blood-vessels, from the ventral region, and eventually applying itself to the walls of the cavity, in which the developing organism is contained, enables these vessels to become the channel by which the stream of nutriment, required to supply the wants of the offspring, is furnished to it by the parent.
Mary Hearn, twelve years of age, was inoculated with matter taken from the arm of Susan Phipps. 6th day: A pustule beginning to appear, slight pain in the axilla. 7th: A distinct vesicle formed. 8th: The vesicle increasing; edges very red; no deviation in its appearance at this time from the inoculated smallpox. 9th: No indisposition; pustule advancing.
The fore-end of the medullary tube expands into a vesicle and forms the brain, which soon divides into five cerebral vesicles. In the sides of it appear the three higher sense-organs: nose, eyes, and auditory vesicles. No jaws, limbs, or floating bladder. The ancestors of these Amniotes develop an amnion and allantois, and gradually assume the mammal, and finally the specifically human, form.
The useless relic of it passes through the wall of the petrous bone in the shape of a narrow canal, and is called the vestibular aqueduct. It is only the inner and lower bulbous part of the separated auscultory vesicle that develops into the highly complex and differentiated structure that is afterwards known as the secondary labyrinth.
They resemble tadpoles in outward appearance, and use their tails as oars, as the tadpoles do. However, this lively and highly-developed condition does not last long. At first there is a progressive development; the foremost part of the medullary tube enlarges into a brain, and inside this two single sense-organs are developed, a dorsal auditory vesicle and a ventral eye.
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