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Just noting that an ancestral origin of this kind is implied by the fact that in low types of Metazoa a hollow sphere of cells is the form first assumed by the unfolding embryo, I draw attention to the point here of chief interest; namely that the primary differentiation of this hollow sphere is in such case determined by a difference in the converse of its parts with the medium and its contents; and that the subsequent invagination arises by a continuance of this differential converse.
Now with the finger, thrust in one side of the ball until it touches the other: so making a cup. This action will stand for the process of invagination. Imagine that by continuance of it, the hemispherical cup becomes very much deepened and the opening narrowed, until the cup becomes a sac, of which the introverted wall is everywhere in contact with the outer wall.
The foremost pair of gill-arches become the maxillary arches, from which we get our upper and lower jaws. A third essential character of the Gnathostomes, that distinguishes them very conspicuously from the lower vertebrates we have dealt with, is the formation of a blind sac by invagination from the fore part of the gut, which becomes in the fishes the air-filled floating-bladder.
The patient may be put at once into a sitting bath or small tub, and a panful of cold water poured or dashed on to the bowels; they then contract so powerfully, and shorten themselves so much, that all invagination, as it is called, is made to cease instantly.
The diseases that appendicitis may be confounded with and must be differentiated from are obstruction, renal colic, hepatic colic, gastritis, enteritis, salpingitis, peritonitis due to gastric or intestinal ulcer, enterolith, obstipation, invagination or intussusception, hernia, external or internal, volvulus, stricture and typhoid fever.
The follicles of these hairs extend down into the derma, and from the upper end of the follicle, i.e. near the aperture of the invagination, a long cellular outgrowth extends down into the derma, branches at its end, and becomes hollow. These branches are the tubules of the future milk gland. Another outgrowth from the follicle forms a sebaceous gland.
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.
We will set out with the fact already noted in passing, that after repeated spontaneous fissions have changed the original fertilized germ-cell into that cluster of cells which forms a gemmule or a primitive ovum, the first contrast which arises is between the peripheral parts and the central parts. Where, as with lower creatures which do not lay up large stores of nutriment with the germs of their offspring, the inner mass is inconsiderable, the outer layer of cells, which are presently made quite small by repeated subdivisions, forms a membrane extending over the whole surface the blastoderm. The next stage of development, which ends in this covering layer becoming double, is reached in two ways by invagination and by delamination; but which is the original way and which the abridged way, is not quite certain. Of invagination, multitudinously exemplified in the lowest types, Mr. Balfour says: "On purely
Hence before and during invagination there is an increasing differentiation of the inner from the outer layer. The animal cells of the outer layer soon develop vibratory hairs; the vegetal cells of the inner layer do so much later. A thread-like process grows out of each cell, and effects continuous vibratory movements.
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