two whites, though seeming to us to be only one, each wrapped up in a membrane of its own, one white within the other. In the midst of all is the yolk, wrapped up likewise in its own membrane. At each end of this are two ligaments, called chalasoe, which are, as it were, the poles of this microcosm, being white dense substances, made from the membranes, and serving to keep the white and the yolk in their places. The cicatricula, which is the part where the animal first begins to show signs of life, is not unlike a vetch or a lentil, lying on one side of the yolk, and within its membrane. All these contribute to the little animal's convenience or support; the outer membranes and ligaments preserve the fluids in their proper places; the white serves as nourishment, and the yolk, with its membranes, after a time, becomes a part of the animal's body. This is a description of a hen's egg, and answers to that of all others, how large or how small soever. Previous to putting the eggs to the hen, our philosophers first examined the cicatricula, or little spot, already mentioned; and which may be considered as the most important part of the egg. This was found in those that were impregnated by the cock to be large; but in those laid without the cock, very small. It was found by the microscope to be a kind of bag, containing a transparent liquor, in the midst of which the embryo was seen to reside. The embryo resembled a composition of little threads, which the warmth of future incubation tended to enlarge by varying and liquifying the other fluids contained within the shell, and thus passing them either into the pores or tubes of their

substance. Upon placing the eggs in a proper warmth, either under the sun or in a stove, after six hours the vital speck begins to dilate, like the pupil of the eye. The head of the chicken is distinctly seen, with the backbone, something resembling a tadpole, floating in its ambient fluid, but as yet seeming to assume none of the functions of animal life. In about six hours more, the little animal is seen more distinctly; the head becomes more plainly visible, and the vertebræ of the back more easily perceivable. All these signs of preparation for life are increased in six hours more; and at the end of twenty-four hours the ribs begin to take their places, the neck begins to lengthen, and the head to turn to one side. At this time, also, the fluids in the egg seem to have changed place; the yolk, which was before in the center of the shell, approaches nearer the broad end. The watery part is in some measure evaporated through the shell, and the grosser part sinks to the small end. The little animal appears to turn towards the part of the broad end, in which a cavity has been described, and with its yolk, seems to adhere to the membrane there. At the end of forty hours the great work of life seems fairly begun, and the animal plainly appears to move; the backbone, which is of a whitish color, thickens; the head is turned still more on one side; the first rudiments of the eye begin to appear; the heart beats; and the blood begins already to circulate. The parts, however, as yet, are fluid; but by degrees, become more and more tenacious, and harden into a kind of jelly. At the end of two days, the liquor in which the chicken

swims, seems to increase; the head appears with two little bladders, in the place of eyes; the heart beats in the manner of every embryo, where the blood does not circulate through the lungs. In about fourteen hours after this, the chicken is grown more strong, its head is, however, still bent downwards; the veins and arteries begin to branch, in order to form the brain; and the spinal marrow is seen stretching along the backbone. In three days the whole body of the chicken appears bent, the head with its two eye-balls, with their different humors, now distinctly appear; and five other vesicles are seen, which soon unite to form the rudiments of the brain. The outlines also of the thighs and wings begin to be seen, and the body begins to gather flesh. At the end of the fourth day, the vesicles that go to form the brain, approach each other; the wings and thighs appear more solid; the whole body is covered with a jelly-like flesh; the heart that was hitherto exposed, is now covered up within the body, by a very thin transparent membrane; and at the same time, the umbilical vessels that unite the animal to the yolk, now appear to come forth from the abdomen. After the fifth and sixth days, the vessels of the brain begin to be covered over; the wings and thighs lengthen; the belly is closed up and tumid; the liver is seen within it very distinctly, not yet grown red, but of a very dusky white; both the ventricles of the heart are discerned, as if they were two separate hearts beating distinctly, the whole body of the animal is covered over; and the traces of the incipient feathers are already to be

seen.

At the seventh day the head appears very

large; the brain is covered entirely over; the bill begins to appear between the eyes; and the wings, thighs and legs have acquired their perfect figure. But towards the end of incubation, the umbilical vessels shorten the yolk, and with it the intestines are thrust up into the body of the chicken by the action of the muscles of the belly; and the two bodies are thus formed into one. During this state, all the organs are found to perform their secretions; the bile is found to be separated as in grown animals, but it is fluid, transparent and without bitterness, and the chicken then appears to have lungs. On the tenth day the muscles of the wings appear, and the feathers begin to push out. On the eleventh, the heart, which hitherto had appeared divided, begins to unite; the arteries which belong to it join into it, like the fingers into the palm of the hand. As the animal thus, by the eleventh day completely formed, begins to gather strength, it becomes more uneasy in its situation, and exerts its animal powers with increasing force. For sometime before it is able to break the shell, in which it is imprisoned, it is heard to chirup, receiving a sufficient quantity of air, for this purpose, from that cavity which lies between the membrane and the shell, and which must contain air to resist the external pressure. At length, upon the twentieth day, in some birds sooner, and later in others, the enclosed animal breaks the shell within which he has been confined, with its beak; and by repeated efforts, at last procures its enlargement, and becomes an organized existence to our senses.'

The resemblance between the beginning animal in

the egg, and the embryo in the womb, is very striking; and this similitude has induced many to assert, that all animals are produced from eggs in the same manner. They consider an egg excluded from the body by some, and separated into the womb by others, to be actions merely of one kind; with this only difference, that the nourishment of the one is kept within the body of the parent, and increases as the embryo happens to want the supply; the nourishment of the other is prepared all at once, and sent out with the beginning animal, as entirely sufficient for its future support. In this investigation, Graaf has, with a degree of patience characteristic of his nation, attended the progress and increase of various animals in the womb, and minutely marked the changes they undergo. Having dissected a rabbit, half an hour after impregnation, he perceived the horns of the womb, that go to embrace and communicate with the ovary, to be more red than before; but no other change in the rest of the parts. Having dissected another six hours after, he perceived the follicles, or the membrane covering the eggs contained in the ovary, to become reddish. In a rabbit dissected after twenty-four hours, he perceived in one of the ovaries three follicles and in the other five, that were changed, having become, from transparent, dark and reddish. In one dissected after three days, he perceived the horns of the womb very strictly to embrace the ovaries; and he observed three of the follicles in one of them, much longer and harder than before; pursuing his inquisition, he also found two of the eggs actually separated into