At a very early period the germ undergoes modifications of form, varying with the department of the animal kingdom to which it belongs. Thus, in the articulata (insects, crustaceans, etc.). the germ is divided into segments, indicating the transverse divisions which occur in the adult animal; while the germ of vertebrate animals displays a longitu dinal furrow, marking the position which the vertebral column and spiral cord are to occupy. The development of this furrow is highly important. At first, the furrow is very shallow, and a minute transparent band appears under it, called the primitive stripe. The walls of the furrow consist of two raised edges formed by a swelling of the germ on both sides of the primitive stripe. As these walls increase in height, their summits approach each other; and after a time they unite and convert the furrow into a closed canal, which is soon filled with a fluid from which the brain and spinal cord are subse quently formed.
The primitive stripe is gradually developed into a cartilaginous structure, termed the chorda dorsalis, which is the representative of the back-bone. In the meantime, the margin of the germ continues to extend over the surface of the yolk,until at length the latter is entirely closed in a cavity thus formed by the germ. In this lower cavity, the intestines and other organs of vegetative life arc subsequently developed.
In all classes of the animal kingdom, the embryo rests upon the yolk, and covers it like a cap; but the direction by which its edges approach each other and unite to form the lower cavity, is very different in different kinds of animals, and is highly distinctive in reference to zoological classification. In all the vertebrata, the embryo lies with its face or ventral surface towards the yolk, and thus the suture or line at which the edges of the germ unite to enclose the yolk, and which in mammals forms the navel, lies in front. Another suture is found along the back, arising from the folding together of the upper surfaces of the germ, to form the dorsal canal. In the affection known as spina btfuta, this suture is incomplete. In the articulata, the embryo lies with its back upon the yolk; in the cephalopoda, the yolk communicates, as in the vertebrata, with the ventral surface of the body, but no dorsal canal is formed in them; and in the other departments of animal life, we find special and characteristic relations between the embryo and the yolk.
° The development of the embryo of vertebrated animals is best observed in the eggs of fishes, as from their transparency they do not require to be cut open; and with due care the whole series of embryonic changes may be observed upon the same individual, and the succession of appearances of the different organs may be ascertained. Prof. .A.gassiz has carefully examined and depicted the principal phases of development in the white-fish, which belongs to the salmon family, and we shall endeavor to give his chief results. The egg when laid is spherical, about the size of a small pea, and nearly
transparent. It contains no albumen, and the shell-membrane and the membrane of the yolk seem fused into one. In a few hours after it has been laid, provided it has come in contact with the fecundating fluid of the male, a separation between these two mem branes takes place, in consequence of the absorption of water, by which the size of the egg is increased. Between the shell-membrane and the yelk there is now a considerable transparent space, corresponding in position to the albumen of birds' eggs. Oil globules are scattered through the yelk, but soon arrange themselves in a stratum or disk towards the upper part, where a swelling, in the shape of a transparent vesicle, begins to show itself. It is composed of very delicate and minute cells, and is the germ in its earliest As the germ increases, and has a larger surface in contact with the yelk, we • notice a depression on its upper part, and soon after a second furrow appears at right angles to the former, so that the germ now presents four elevations. A continuous subdivision of this nature goes on during the second and third days, until the germ is divided into numerous minute spheres, which form what is termed the mulberry mass. This appearance, however, does not long continue; at the end of the third day, the fissures disappear, and the germ continues to extend as an enVelope around the yelk, which it at last entirely ineloses.
On the tenth day, the first outlines of the embryo begin to appear, and we soon dis tinguish in it a depression between two little ridges, whose edges approach till they form a canal. At about the same. time, an enlargement at one end of the furrow is observed, which is the rudimentary head, in which may soon be distinguished traces of the division of the brain corresponding to the organs of sight, hearing, and smell.
Towards the thirteenth day, we see a transparent cartilaginous cord (the chorda dm- 80141) iu the position afterwards occupied by the backbone, composed of large cells, in whiqh transverse divisions are forming. In sonic fishes, as the sturgeon, this cartilagi nous or embryonic state is persistent through life, and no true osseous vertebral column is ever formed. The rudiments of the eye soon appear in the form of a fold in the external membrane of the germ, iu.which the crystalline lens is afterwards formed. At the same time we see at the posterior part of the head an elliptical vesicle, which is the rudimentary car. At this period, the distinction between the upper (serous) and lower (mucous) layers of the germ is best traced; all the changes which have been mentioned occurring in the upper layer.