ON THE INCUBATED EGG.
The various vital processes of nutrition and formation, which are carried on in the foetus of the mammalia while in its mo ther's body, and by means of the most intimate connection with the parent, are effected in the incubated chick by its own powers, quite independently of the mother, and without any extraneous as sistance, except that of the atmospheric air and a certain degree of warmth.
The egg is covered within the shell, by a white and firm membrane (mem brana albuminis) which contains no blood vessels. The twolayers ofthis membrane, which in other parts adhere closely to each other, leave at the large end a space which is filled with atmospheric air.
This membrane includes the two whites of the egg, each of which is sur rounded by a delicate membrane. The external of these is the most fluid and transparent ; the inner one thicker and more opaque ; they may be separated in eggs which are boiled hard.
The internal white surrounds the yolk, which is contained in a peculiar mem brane called the yolk-bag. From each end of this proceeds a white knotty body, which terminates in a flocculent extre mity in the albumen. These are called the chalazx, or grandines.
A small round milk-white spot, called the tread of the cock (cicatricula, or ma cula), is formed on the surface of the yolk-bag. It is surrounded by one or more whitish concentric circles (halones, or circuli), the use of which, as well as that of the cicatricula itself, and of the chalazx, is not yet ascertained.
We now proceed to notice the wonder ful successive changes which go on during the incubation of the egg, and the meta morphoses which are observed, b oth in the general form of the chick and in particu lar viscera. The periods of these changes will be set down from the hen, as afford ing the most familiar example. It will be best to give, first, a cursory chronological view of the whole process, and then to make a few remarks on some of the most important parts of the subject.
A small shining spot, of an elongated form, with rounded extremities, but nar rowest in the middle, is perceived at the end of the first day, not in nor upon the cicatricula, bet very near that part on the yolk-bag (nidus pulli ; colliquamentum ; areola pellucida.) This may be said to
appear before-hand, as the abode of the chick which is to follow.
No trace of the latter can be discerned before the beginning of the second day; and then it has an incurvated form, resem bling a gelatinous filament with large ex tremities, very closely surrounded by the amnion, which at first can scarcely be dis tinguished from it.
About this time the halones enlarge their circles, but they soon after disappear entirely, as well as the cicatricula.
The first appearance of red blood is discerned on the surface of the yolk-bag towards the end of the second day. A series of points is observed which form grooves ; and these, closing, constitute vessels, the trunks of which become con. nected to the chick. The vascular sur face itself is called figura venosa, or area vasculosa ; and the vessel, by which its margin is defined, vena terminalis. The trunk of all the veins joins the vena por tz ; while the arteries, which ramify on the yolk-bag, arise from the mesenteric artery of the chick.
On the commencement of the thirdday, the newly-formed heart(the primaryorgan of the circulating process which now com mences) is discerned by means of its triple pulsation,and constitutes a threefold punc. turn saliens. Some parts of the incubated chicken are destined to undergo succes sive alterations in their form ; and this holds good of the heart in particular. In its first formation it resembles a tortuous canal, and consists of three dilatations ly ing close together, and arranged in a tri angle. One of these, which is properly the right, is then the common auricle ; the other is the only ventricle, but after. wards the left ; and the third is the dilat ed part of the aorta (bulbus aorta.) About the same time, the spine, which was originally extended in a straight line, becomes incurvated; and the distinction of the vertebrm is very plain. The eyes may be distinguished by their black pigment, and comparatively immense size ; and they are afterwards remarkable, in conse quence of a peculiar slit in the lower part of the iris.