tion of an embryo has commenced. It takes place in all species of animals, although it varies in detail according to the special constitution of the egg, and the presence or absence of accessory parts. In all the mammalia, as well as in many of the invertebrates, where the vitellus is very small, and where the body of the embryo immediately after its formation is to be supplied with nourishment from without, the process is that described above. In the birds, in scaly reptiles, and in many fish, where the vitellus or yolk is of large size, and contains additional nutritive matter, segmentation takes place only in a thin layer which occupies the surface of the great mass of the yolk; and, beginning at one spot, extends thence from within outward, so that it advances more rapidly at the centre of the segmenting region than at its periphery. But in all cases segmentation of the vitellus is the first change to occur in the process of development, and its result is always the same, namely, to divide the vitellus, which was at first of uniform texture throughout, into a great number of minute bodies, which soon present the character of animal cells.

Blastoderm, or Germinal Membrane.-The cells which are formed, in the manner above described, by the segmentation of the vitellus, become more closely packed as they increase in number; and finally, by their mutual contact, and adhesion at their adjacent edges, they serve to form a continuous organized membrane, known as the germinal membrane or blastoderm.

During the formation of this membrane, moreover, the egg, while passing through the Fallopian tube, increases in size. The albuminous matter with which it is enveloped becomes liquefied; and, being absorbed by endosmosis through the vitelline membrane, furnishes the material for the more solid and extensive growth of the newly-formed structures. A certain quantity of fluid also accumulates in the central cavity of the egg.

The next change which takes place consists in the division or splitting of the blastoderm into two layers, which are known as the external and internal blastodermic layers. They are both still composed exclusively of cells; but those of the external layer are smaller and more compact, while those of the internal are larger and less consistent. The egg then has the form of a globular sac, the walls of which consist of three concentric layers, lying in contact with and inclosing each other, namely: 1st, the structureless vitelline membrane on the outside; 2d, the external blastodermic layer, composed of cells; and 3d, the internal blastodermic layer, also composed of cells. The cavity of the egg is occupied by an albuminous fluid, absorbed from the exterior and destined to serve as nutritious material.

It is by this process that the simple globular mass of the vitellus is converted into an organized structure. For the blastoderm, although consisting only of cells which are nearly uniform in size and shape, is nevertheless an organized membrane, made up of anatomical elements. It is the first sign of distinct organization which makes its appearance

in the egg; and as soon as it is completed, the body of the fœvas is formed. The blastoderm is, in fact, the foetus in its earliest condition; for although its texture is at this time exceedingly simple, all the various organs of the body will afterward be produced from it by the modificstion of its different parts. The further process of formation is comparatively simple in some classes of animals, more complicated in others; and its general features are most easily understood by commencing with the study of embryonic development as it takes place in the frog.

Formation of Organs in the Embryo.-The egg of the frog, when discharged and fecundated, is deposited in the water, enveloped in an elastic cushion of albuminous matter. It is thus freely exposed to the light, the air, and the moderate warmth of the sun's rays, and is supplied with an abundance of moisture and appropriate nutritious material. Its development is distinguished accordingly by a character of great simplicity; since the whole of the vitellus is directly converted into the body of the embryo. There are no accessory organs required, and consequently no complications of the formative process.

The two blastodermic layers, above described, represent together the commencement of the body of the embryo. They serve, however, for the production of two different systems; and the entire process of their development may be expressed as follows: The external blastodermic layer produces the skin, the cerebro-spinal axis, and the organs of animal life; while the internal layer produces the mucous membrane of the alimentary canal, and the organs of nutrition.

The first sign of advancing organization in the external blastodermic layer shows itself in a thickening and condensation of its structure. The thickened portion has the form of an elongated oval spot, termed

Fig. 241.

the "embryonic spot" (Fig. 241), the wide edges of which are somewhat more opaque than the rest of the blastoderm. 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, called the "primitive trace."

In the anterior portion of the area pellucida, the substance of the blastoderm rises up in such a manner as to form two nearly parallel ridges or plates, which approach each other, from side to side, over what will be the dorsal aspect of the embryo, and are therefore called the "dorsal plates." Between them is included a groove, termed the "medullary groove." The dorsal plates gradually meet each other and coalesce upon the median line, thus converting the intervening groove into a canal. coalescence of the edges of the two dorsal plates takes place first in the

anterior part of the area pellucida and extends gradually backward; and when it is complete throughout their length, the whole of the medullary groove has been converted into a closed canal. This is the "medullary canal ;" and in its cavity will afterward be formed the cerebro-spinal axis, by a growth of nervous matter from its internal surface. At its anterior extremity, the medullary canal is large and rounded, to accommodate the brain and the medulla oblongata; its remainder is narrow, and pointed posteriorly, and is destined to contain the spinal cord.

In a diagrammatic section of the egg at this stage, made transversely to the longitudinal axis of the embryo (Fig. 242), the dorsal plates may be seen approaching each other above, on each side of the medullary groove. At a more advanced period (Fig. 243) they are fairly united with each other, and inclose the cavity of the medullary canal. At

the same time, the edges of the thickened portion of the blastoderm grow outward and downward, extending over the lateral portions of the vitelline mass. These are called the "abdominal plates;" and, as they enlarge, they tend to approach each other below and inclose the abdominal cavity, as the dorsal plates united above, and inclosed the medullary canal. At last the abdominal plates actually unite on the median line (at 1, Fig. 243), embracing the whole of the internal blastodermic layer (5), which incloses in turn the remains of the original vitellus and the albuminous fluid accumulated in its cavity.

During this time, there is formed, in the thickened central part of the blastoderm, immediately beneath the medullary canal, a longitudinal cartilaginous cord, the "chorda dorsalis." A round the chorda dorsalis

are afterward developed the bodies of the vertebræ (Fig. 243, 4), 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.

In a longitudinal section of the egg, made while this process is going on, the thickened portion of the external blastodermic layer (Fig. 244, 1) may be seen in profile. The anterior portion (2), which will form the head, is thicker than the posterior (3), which will form the tail. As the whole mass grows rapidly, both in the anterior and the posterior direction, the head becomes thick and voluminous, while the tail begins to project backward, and the egg assumes an elongated form. (Fig. 245.) Fig. 244.

Fig. 245.

Diagram of FROG'S EGG, in an early stage of development; longitudinal section.-1. Thickened portion of external blastodermic layer. 2. Anterior extremity of the embryo. 3. Posterior extremity. 4. Internal blastodermic layer. 5. Cavity of vitellus.

ment.

The abdominal plates also meet upon its under surface, and complete the closure of the abdominal cavity. The internal blastodermic layer is seen, in the longitudinal section of the egg, embraced by the abdominal plates, and inclosing, as before, the remains of the vitellus.

As development goes on (Fig. 246), the head becomes larger, and shows traces of the formation 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, to form the brain and medulla oblongata. In the mean time, the internal blastodermic layer, which is subsequently converted into the intestinal canal, has been shut

in by the abdominal walls, and still forms a closed sac, of slightly elongated figure, without inlet or outlet. Afterward, the mouth is formed by means of a 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.

By a continuation of the same process, the different portions of the external blastodermic layer are further developed, resulting in the complete formation of the various parts of the skeleton, the integument, the organs of special sense, and the voluntary muscles and nerves. The tail at the same time acquires sufficient size and strength to be capable of acting as an organ of locomotion. (Fig. 247.) The intestinal canal,

which has been formed from 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 curvilinear folds.

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 in length and becomes spirally coiled up in the abdominal cavity, attaining 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 animal. The posterior limbs are the first to make their appearance, by budding or sprouting from the sides of the body at the base of the tail. (Fig. 248.) The anterior extremities are for a time concealed beneath the integument, but afterward become liberated, and show themselves externally. At first both the fore and hind legs are very small, incompiete in structure, and useless for purposes of locomotion. They soon, however, increase in size and strength; while 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