At or soon after the seventeenth day, the lower (or mucous) layer separates into two laminte, the inferior of which becomes the intestine. The heart at about the same time appears in the form of a simple cavity, in the midst of a mass of cells belonging to the middle or vascular layer. As soon as the cavity of the heart is closed in, regular motions of contraction and expansion are observed, and the movement of blood-corpuscles within it is seen.
It is not till the thirtieth day that any traces of a circulation of the blood are mani fested; we then notice two currents, one running towards the head, and the other towards the trunk, with correspondino. returning currents. The traces of the liver are now seen. The embryo begins to liberate itself at both extremities from the yelk, the tail first becoming free, and moving in violent jerks.
The embryo, although still inclosed in the egg-membranes, now unites all the essen tial conditions for the exercise of the functions of animal life: it has a brain, an intes tine, a pulsating heart, and circulating blood, and a limited amount of spontaneous motion; but the forms of the organs are still incomplete, nor have they acquired the precise shape characterizing the class to which it belongs. The embryo is as yet only a vertebrate animal in general, and might be taken for that of a frog as well as for that of at fish.
Towards the close of the embryonic period, after the fortieth day, the embryo acquires a more definite shape: the jaws protrude; the nostrils approach the end of the snout; divisions are observed in the fin-like structure which surrounds the body; the anterior extremities, which were mere knobs, become rudimentary pectoral fins; and, finally, the gill-openings appear, so that the fish-type is now obvious. In this state, the
young white fish escapes from the egg about the sixtieth day after it is laid; but its development is still incomplete. We cannot yet tell to what genus the fish belongs. The gill-covers are not yet formed, there are no teeth, the mouth is below instead of the most projecting part of the head, and the fins have no rays.' The remainder of the yelk is suspended from the belly in the form of a bladder, but it daily diminishes in size, till it is at length completely absorbed by the animal. The duration of these changes varies extremely in different fishes; some accomplish them in a few days, while in others, months are required.
In frogs and all the naked reptiles, the development is very similar to that of fishes; in the scaly reptiles (snakes, lizards, turtles), audit) birds, there are peculiar membranes surrounding and protecting the embryo during its growth, termed the allantois and the amnios; and in mammals there are additional complications of structure, which it would be impossible to describe clearly without occupying more space than could be allotted to a subject of this nature. For further information on this subject, we.may..refer,to the various writings of Agassiz, especially his Lectures on Coniparative Embryology, and his Comparative Physiology, from which this article is mainly drawn, and to Carpenter's Comparative Physiology.