At the end of three weeks

One month

Two months

CORPUS LUTEUM OF MENSTRUATION.

CORPUS LUTEUM OF PREGNANCY.

Three-quarters of an inch in diameter; central clot reddish; con-
voluted wall pale.

Smaller; convoluted wall bright | Larger; convoluted wall bright
yellow; clot still reddish.
Reduced to the condition of an
insignificant cicatrix.

yellow; clot still reddish. Seven-eighths of an inch in diameter; convoluted wall bright yellow; clot perfectly decolorized.

Still as large as at end of second month; clot fibrinous; convoluted wall paler.

One-half an inch in diameter; central clot converted into a radiating cicatrix; the external wall tolerably thick and convoluted, but without any bright yellow color,

CHAPTER VII.

ON THE DEVELOPMENT OF THE IMPREGNATED EGG -SEGMENTATION OF THE VITELLUS-BLASTODERMIC MEMBRANE-FORMATION OF ORGANS IN THE FROG.

WE have seen, in the foregoing chapters, how the egg, produced in the ovarian follicle, becomes gradually developed and ripened, until it is ready to be discharged. The egg, accordingly, passes through several successive stages of formation, even while still contained within the ovary; and its vitellus becomes gradually completed, by the formation of albuminous material and the deposit of molecular granulations. The last change which the egg undergoes, in this situation, and which marks its complete maturity, is the disappearance of the germinative vesicle. This vesicle, which is, in general, a prominent feature of the ovarian egg, disappears but a short time previous to its discharge, or even just at the period of its leaving the Graafian follicle.

The egg, therefore, consisting simply of the mature vitellus and the vitelline membrane, comes in contact, after leaving the ovary, and while passing through the Fallopian tube, with the spermatic fluid, and thereby becomes fecundated. By the influence of fecundation, a new stimulus is imparted to its growth; and while the vitality of the unimpregnated germ, arrived at this point, would have reached its termination, the fecundated egg, on the contrary, starts upon a new and more extensive course of development, by which it is finally converted into the body of the young animal.

The egg, in the first place, as it passes down the Fallopian tube, becomes covered with an albuminous secretion. In the birds, as we have seen, this secretion is very abundant, and is deposited in successive layers around the vitellus. In the reptiles, it is also poured out in considerable quantity, and serves for the nourishment of the egg during its early growth. In quadrupeds, the albuminous matter is supplied in the same way, though in smaller quantity, by the

mucous membrane of the Fallopian tubes, and envelopes the egg in a layer of nutritious material.

A very remarkable change now takes place in the impregnated egg, which is known as the spontaneous division, or segmentation, of the vitellus. A furrow first shows itself,

running round the globular mass of the vitellus in a vertical direction, which gradually deepens until it has divided the vitellus into two separate halves or hemispheres. (Fig. 177, a.) Almost at the same time another furrow, running at right angles with the first, penetrates also the substance of the vitellus and cuts it in a transverse direction. The vitellus is thus divided into four equal portions (Fig. 177, b), the edges and angles of which are rounded off, and which are still contained in the cavity of the vitelline membrane. The spaces between them and the internal surface of the vitelline membrane are occupied by a transparent fluid.

The process thus commenced goes on by a successive formation of furrows and sections, in various directions. The four vitelline segments already produced are thus subdivided into sixteen, the sixteen into sixtyfour, and so on; until the whole vitellus is converted into a mulberry shaped mass, composed of minute, nearly spherical bodies, which are called the "vitelline spheres." (Fig. 177, c.) These vitelline spheres have a somewhat firmer consistency than the original substance of the vitellus; and this consistency appears to increase, as they successively multiply

Fig. 177.

in numbers and diminish in size. At last they have become so abundant as to be closely crowded together, compressed into polygonal forms, and flattened against the internal surface of the vitel

line membrane. (Fig. 177, d.) They have by this time been converted into true animal cells; and these cells, adhering to each other by their adjacent edges, form a continuous organized membrane, which is termed the Blastodermic membrane.

During the formation of this membrane, moreover, the egg, while passing through the Fallopian tubes into the uterus, has increased in size. The albuminous matter with which it was enveloped has liquefied; and, being absorbed by endosmosis through the vitelline membrane, has furnished the materials for the more solid and extensive growth of the newly-formed structures. It may also be seen that a large quantity of this fluid has accumulated in the central cavity of the egg, inclosed accordingly by the blastodermic membrane, with the original vitelline membrane still forming an external envelope round the whole.

The next change which takes place consists in the division or splitting of the blastodermic membrane into two layers, which are known as the external and internal layers of the blastodermic membrane. They are both still composed exclusively of cells; but those of the external layer are usually smaller and more compact, while those of the internal are rather larger and looser in texture. The egg then presents the appearance of a globular sac, the walls of which consist of three concentric layers, lying in contact with and inclosing each other, viz., 1st, the structureless vitelline membrane on the outside; 2d, the external layer of the blastodermic membrane, composed of cells; and 3d, the internal layer of the blastodermic membrane, also composed of cells. The cavity of the egg is occupied by a transparent fluid, as above mentioned.

This entire process of the segmentation of the vitellus and the formation of the blastodermic membrane is one of the most remarkable and important of all the changes which take place during the development of the egg. It is by this process that the simple globular mass of the vitellus, composed of an albuminous matter and oily granules, is converted into an organized structure. For the blastodermic membrane, though consisting only of cells nearly uniform in size and shape, is nevertheless a truly organized membrane, made up of fully formed anatomical elements. It is, moreover, the first sign of distinct organization which makes its appearance in the egg; and as soon as it is completed, the body of the new foetus is formed. The blastodermic membrane is, in fact, the body of the foetus. It is at this time, it is true, exceedingly simple in texture; but we shall see hereafter that all the future organs

of the body, however varied and complicated in structure, arise out of it, by modification and development of its different parts.

The segmentation of the vitellus, moreover, and the formation of the blastodermic membrane, take place in essentially the same manner in all the different classes of animals. It is always in this way that the egg commences its development, whether it be destined to form afterward a fish or a reptile, a bird, a quadruped or a man. The peculiarities belonging to different species show themselves afterward, by variations in the manner and extent of the development of different parts. In the higher animals and in the human subject the development of the egg becomes an exceedingly complicated process, owing to the formation of various accessory organs, which are made requisite by the peculiar conditions under which the development of the embryo takes place. It is, in fact, impossible to describe or understand properly the complex embryology of the quadrupeds, and more particularly that of the human subject, without first tracing the development of those species in which the process is more simple. We shall commence our description, therefore, with the development of the egg of the frog, which is for many reasons particularly appropriate for our purpose.

The egg of the frog, when discharged from the body of the female and fecundated by the spermatic fluid of the male, is deposited in the water, enveloped in a soft elastic cushion of albuminous substance. It is therefore in a situation where it is freely exposed to the light, the air, and the moderate warmth of the sun's rays, and where it can absorb directly an abundance of moisture and of ap propriate nutritious material. We find accordingly that under these circumstances the development of the egg is distinguished 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 complication of the formative process.

The two layers of the blastodermic membrane, above described, represent together the commencement of all the organs of the foetus. They are intended, however, for the production of two different systems; and the entire process of their development may be expressed as follows: The external layer of the blastodermic membrane produces the spinal column and all the organs of animal life; while the internal layer produces the intestinal canal, and all the organs of vegetative life.

The first sign of advancing organization in the external layer of