EXERCISE THE TWENTY-SECOND.
The inspection after the fourteenth day.
From the seventh to the fourteenth day everything has grown and become more conspicuous. The heart and all the other viscera have now become concealed within the abdomen of the embryo, and the parts that formerly were seen naked and projecting externally, can now only be perceived when the thorax and abdomen are laid open. The chick too now begins to be covered with feathers, the roots of which are first perceived as black points. The pupils of the eyes are distinguished; the eyelids appear, as does also the membrana nictitans in the greater canthus of the eye, a membrane which is proper to birds, and which they use for cleansing the eyeball. The convolutions of the brain farther make their appearance; the cerebellum is included within the skull; and the tail acquires the characteristic shape of the bird’s rump.
After the fourteenth day the viscera, which up to this time have been white, gradually begin to assume a flesh or reddish colour. The heart, having now entered the penetralia of the thorax and been covered with the sternum, inhabits the dwelling place which itself had formed. The cerebrum and cerebellum acquire solidity under the dome of the skull; the stomach and intestines, however, are not yet included within the abdomen, but, connected with the parts within, hang pendulous externally.
Of the two vessels that proceed from the abdomen to the umbilicus, near the anus, one is an artery, as its pulse proclaims, and arises from the arteria magna or aorta, the other is a vein, and extends from the vitellus by the side of the intestines to the vena portæ, situated in the concave part of the liver. The other trunk of the umbilical vessels, collecting its branches from the albumen, passes the convexity of the liver, and enters the vena cava near the base of the heart.
As all these things go on becoming clearer from day to day, so the greater portion of the albumen is also gradually consumed; this, however, is nowise the case with the vitellus, which remains almost entire up to this time, and indeed is seen of the same size as it was the first day.
In the course of the following days five umbilical vessels are conspicuous; one of these is the great vein, arising from the cava above the liver, and distributing its branches to the albumen; two other veins proceed from the porta, both having the same origin, and run to the two portions of the vitellus, which we have but just described; and these are accompanied by two arteries arising one on either side from the lumbars.
The chick now occupies a larger space in the egg than all the rest of the matter included in it, and begins to be covered with feathers; the larger the embryo grows, the smaller is the quantity of albumen that is present. It is also worthy of observation, that the membrane of the colliquament which we have said unites with the external investing membrane, and constitutes the secundine or chorion, now includes the whole of the vitellus in one, and becoming contracted, draws the vitellus along with the intestines towards the chick, conjoins them with its body, and incloses them as it were in a thick sac. Everything that was previously extremely delicate and transparent, becomes more opaque and fleshy as the sac contracts, which at length, like a hernial tumour of the scrotum, includes and supports both the intestines and the yelk; contracting every day in a greater and greater degree, it comes finally to constitute the abdomen of the chick. You will find the yelk, about the eighteenth day, lying [in its bag] among the intestines, the belly at large being lax; yet are the parts not so firmly fixed but that the intestines (as in the case of a scrotal hernia), along with the vitellus, can be pushed up into the belly, or forced out of it as it were into a pouch. I have occasionally seen the vitellus prolapsed in this way from the abdominal cavity of a pigeon, which had been prematurely excluded from the shell in the summer season.
The chick at this epoch looks big-bellied and as if it were affected with a hernia, as I have said. And now the colliquament, which was at first in large quantity, gradually grows turbid, suffers change, and is consumed, so that the chick comes to lie bent over the vitellus. At the same period, before the liver assumes its sanguineous colour, and performs the business of what is called the second concoction, the bile, which is commonly believed to be separated as an excretion by the power of the liver, is seen of a green colour between the lobes of that organ. In the cavity of the stomach there is a limpid fluid contained, obviously of the same appearance and taste as the colliquament in which the fœtus swims; this passing on by the intestines, gradually changes its colour, and is converted into chyle; and finally in the lower portion of the bowels an excrementitious matter is encountered, of the same character as that which is met with in the lower intestines of chicks already excluded from the egg. When the chick is further advanced you may even see this fluid concocted and coagulated; just as in those animals that feed on milk, a coagulum is formed, which afterwards separates into serum and firmer curd.
When the albumen is almost all removed, and only a very small quantity of the colliquament is left, for several days before the exclusion, the chick no longer swims, but, as I have said, bends over the vitellus; and rolled up into a round ball, with the head for the most part placed between the right thigh and wing, it is seen with its beak, nails, feathers, and all other parts complete. Sometimes it sleeps, and sometimes it wakes, and moving about it breathes and chirps. If you apply the egg to your ear, you will hear the chick within making a noise, kicking, and unquestionably chirping; according to Aristotle, he now also uses his eyes. If you cautiously drop the egg into warm water, it will swim, and the chick within, aroused by the warmth, will leap, and, as I have already said, cause the egg to tumble about. And it is by this means that our country folks distinguish prolific from unproductive eggs which sink when put into water.
When the albumen is entirely gone, just before the exclusion, the umbilical vessel, which we have described as distributed to the albumen, is obliterated; or as Aristotle says,[191] “that umbilicus which proceeds to the external secundines is detached from the animal and dies; but the one which leads to the vitellus becomes connected with the small intestine of the chick.”
The excrement that is first formed in the intestines is white and turbid, like softened egg-shell; and some of the same matter may be found contained in the secundines. The philosopher admits this when he says: “At the same time, too, the chick discharges a large quantity of excrement into the outer membrane; and there are white excrements within the abdomen, as well as those that have been evacuated.”
Time running on, very shortly before the exclusion, light green fæces are formed, similar to those which the chick discharges when excluded from the egg. In the crop, too, we can discover a portion of the colliquament which has been swallowed; and in the stomach some curd or coagulum.
Up to this time the liver has not yet acquired its purple or blood-red colour, but has a tint verging from white into yellow, such as the liver of fishes presents. The lungs, however, are of a florid red.
The yelk is now contained in the abdomen among the intestines: and this is the case not merely whilst the chick is in the egg, but even after its exclusion, and when it is running about following its mother in search of food. So that what Aristotle frequently asserts appears to be absolutely true, viz., that the yelk is destined for the food of the chick; and the chick does certainly use it for food, included in his interior as it is, during the few first days after his exclusion, and until such time as his bill gains the hardness requisite to break and prepare his food, and his stomach the strength necessary to digest it. And, indeed, the yelk of the egg is very analogous to milk. Aristotle gives us his support in this opinion in the place already so frequently referred to:[192] “The chick now lies over much of the yellow, which at last diminishes, and, in process of time, disappears entirely, being all taken into the body of the bird, where it is stored, so that on the tenth day after the exclusion of the chick, if the belly be laid open, you will still find a little of the yelk upon the intestines.” I have myself found certain remains of the yelk even upon the thirteenth day; and if the argument derivable from the duct of the umbilical veins which we have described as terminating in the porta of the liver by one or another trunk, be of any avail, the chick is already nourished almost in the same manner as it is subsequently, the sustenance being attracted from the yelk by the umbilical vessels, in the same way as chyle is by and by transmitted by the mesenteric veins from the intestines. For the vessels terminate in either case in the porta of the liver, to which the nourishment attracted in the same way is in like manner transmitted. It is not necessary, therefore, to have recourse to any lacteal vessels of the mesentery, which, in the feathered tribes, are nowhere to be distinguished.
Let me be permitted here to add what I have frequently found: With a view to discovering more distinctly the relative situations of the embryo and the fluids, I have boiled an egg hard, from the fourteenth day of the incubation up to the day when the exclusion would have taken place, the major part of the albumen being already consumed, and the vitellus divided. Breaking the shell, and regarding the position of the chick, I found both the remains of the albumen and the two portions of the vitellus (which we have said are divided by the colliquation induced by the gentle heat), possessing the consistency, colour, taste, and other qualities which distinguish the yelks of unincubated eggs similarly boiled. I have, therefore, frequently asked myself how it came to pass that unprolific eggs set under a hen are made to putrefy and become offensive by the same extraneous heat which produces no such effect upon prolific eggs, both of the fluids of which remain sweet and unchanged, although they have an embryo in the midst of them, (and this even containing some small quantity of excrementitious matter within it,) so that did any one eat the yelk of such an egg in the dark, he would not distinguish it from that of a fresh egg which had never been sat upon.