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the blastodermic membrane shows itself in a thickening and condensation of its structure. This thickened portion has the form of an elongated oval-shaped spot, termed the "embryonic spot" (Fig. 178),

Fig. 178.

IMPREGNATED EGG, with com

the wide edges of which are somewhat more opaque than the rest of the blastodermic membrane. Inclosed within these opaque edges is a narrower colorless and transparent space, the "area pellucida," and in its centre is a delicate line, or furrow, running longitudinally from front to rear, which is called the "primitive trace."

On each side of the primitive trace, in the area pellucida, the substance of

mencement of formation of embryo: the blastodermic membrane rises up in showing embryonic spot, area pellucida, and primitive trace. such a manner as to form two nearly parallel vertical plates or ridges, which approach each other over the dorsal aspect of the foetus and are therefore called the "dorsal plates." They at last meet on the median line, so as to inclose the furrow above described and convert it into a canal. This afterward becomes the spinal canal, and in its cavity is formed the spinal cord, by a deposit of nervous matter upon its internal surface. At the anterior extremity of this canal, its cavity is large and rounded, to accommodate the brain and medulla oblongata; at its posterior extremity it is narrow and pointed, and contains the extremity of the spinal cord.

In a transverse section of the egg at this stage (Fig. 179), the dorsal plates may be seen approaching each other above, on each side of the primitive furrow or "trace." At a more advanced period (Fig. 180) they may be seen fairly united with each other, so as to inclose the cavity of the spinal canal. At the same time, the edges of the thickened portion of the blastodermic membrane grow outward and downward, so as to spread out more and more over the lateral portions of the vitelline mass. These are called the "abdominal plates;" and as they increase in extent they tend to unite with each other below and inclose the abdominal cavity, just as the dorsal plates unite above, and inclose the spinal canal. At last the abdominal plates actually do unite with each other on the median line (at 1, Fig. 180), embracing of course the whole internal layer of the blastodermic membrane (5), which incloses in

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its turn the remains of the original vitellus and the albuminous fluid which has accumulated in its cavity.

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During this time, there is formed, in the thickness of the external blastodermic layer, immediately beneath the spinal canal, a longitudinal cartilaginous cord, called the "chorda dorsalis." Around the chorda dorsalis are afterward developed the bodies of the vertebræ (Fig. 180, 4), forming the chain of the vertebral column; and the oblique processes of the vertebræ run upward from this point into the dorsal plates; while the transverse processes, and ribs, run outward and downward in the abdominal plates, to encircle more or less completely the corresponding portion of the body.

If we now examine the egg in longitudinal section, while this process is going on, the thickened portion of the external blastoder mic layer may be seen in profile, as at 1, Fig. 181. The anterior portion (2), which will form the head, is thicker than the posterior (3), which will form the tail of the young animal. As the whole mass grows rapidly, both in the anterior and posterior direction, the head becomes very thick and voluminous, while the tail also begins to project backward, and the whole egg assumes a distinctly elongated form. (Fig. 182.) The abdominal plates at the same time meet upon its under surface, and the point at which they finally

unite forms the abdominal cicatrix or umbilicus. The internal blastodermic layer is seen, of course, in the longitudinal section of the

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egg, as well as in the transverse, embraced by the abdominal plates, and inclosing, as before, the remains of the vitellus.

As the development of the above parts goes on (Fig. 183), the head becomes still larger, and soon shows traces of the formation

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of organs of special sense. The tail also increases in size, and projects farther from the posterior extremity of the embryo. The spinal cord runs in a longitudinal direction from front to rear, and its anterior extremity enlarges, so as to form the brain and medulla oblongata. In the mean time, the internal blastodermic layer, which is subsequently to be converted into the intestinal canal, has been shut in by the abdominal walls, and still forms a perfectly closed sac, of a slightly elongated figure, without either inlet or outlet. Afterward, the mouth is formed by a process of atrophy and perforation, which takes place through both external and internal layers, at the anterior extremity, while a similar perforation, at the posterior extremity, results in the formation of the anus.

All these parts, however, are as yet imperfect; and, being merely in the course of formation, are incapable of performing any active function.

By a continuation of the same process, the different portions of the external blastodermic layer are further developed, so as to result in the complete formation of the various parts of the skeleton, the integument, the organs of special sense, and the voluntary nerves and muscles. The tail at the same time acquires sufficient size and strength to be capable of acting as an organ of locomotion. (Fig. 184.) The intestinal canal, which has been formed from

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the internal blastodermic layer, is at first a short, wide, and nearly straight tube, running directly from the mouth to the anus. It soon, however, begins to grow faster than the abdominal cavity which incloses it, becoming longer and narrower, and is at the same time thrown into numerous convolutions. It thus presents a larger internal surface for the performance of the digestive process.

Arrived at this period, the young tadpole ruptures the vitelline membrane, by which he has heretofore been inclosed, and leaves the cavity of the egg. He at first fastens himself upon the remains of the albuminous matter deposited round the egg, and feeds upon it for a short period. He soon, however, acquires sufficient strength and activity to swim about freely in search of other food, propelling himself by means of his large, membranous, and muscular tail. The alimentary canal increases very rapidly in length and becomes spirally coiled up in the abdominal cavity, so as to attain a length from seven to eight times greater than that of the entire body.

After a time, a change takes place in the external form of the young animal. Anterior and posterior extremities or limbs begin to show themselves, by budding or sprouting from the corresponding regions of the body. (Fig. 185.) At first these organs are very small, imperfect in structure, and altogether useless for purposes of

locomotion. They soon, however, increase in size and strength; and while they keep pace with the increasing development of the whole body, the tail on the contrary ceases to grow, and becomes shrivelled and atrophied. The limbs, in fact, are destined finally to replace the tail as organs of locomotion; and a time at last arrives (Fig. 186) when the tail has altogether disappeared, while Fig. 185.

Fig. 186.

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the legs have become fully developed, muscular and powerful. Then the animal, which was before confined to an aquatic mode of life, becomes capable of living also upon land, and a transformation is effected from the tadpole into the perfect frog.

During the same time, other changes of an equally important character have taken place in the internal organs. The tadpole at first breathes by gills; but these organs subsequently become atrophied and disappear, being finally replaced by well developed lungs. The structure of the mouth, also, of the integument, and of the circulatory system, is altered to correspond with the varying conditions and wants of the growing animal; and all these changes, taking place in part successively and in part simultaneously, bring the animal at last to a state of complete formation.

The process of development may then be briefly recapitulated as follows:

1. The blastodermic membrane, produced by the segmentation of the vitellus, consists of two cellular layers, viz., an external and an internal blastodermic layer.

2. The external layer of the blastodermic membrane incloses by its dorsal plates the cerebro-spinal canal, and by its abdominal plates the abdominal or visceral cavity.

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