EXERCISE THE SIXTY-FIRST.
Of the uses of the other parts of the egg.
The shell is hard and thick that it may serve as a defence against external injury to the fluids and the chick it includes. It is brittle, nevertheless, particularly towards the blunt end, and as the time of the chick’s exclusion draws near, doubtless that the birth may suffer no delay. The shell is porous also; for when an egg, particularly a very recent one, is dressed before the fire, it sweats through its pores. Now these pores are useful for ventilation; they permit the heat of the incubating hen to penetrate more readily, and the chick to have supplies of fresh air; for that it both breathes and chirps in the egg before its exclusion is most certain.
The membranes serve to include the fluids, and therefore are they present in the same number as these, and therefore is the colliquament also invested, as soon as it is produced, with a tunica propria, which Aristotle[322] refers to in these words: “A membrane covered with ramifications of blood-vessels already surrounds the clear liquid,” &c. But the exit of the chick being at hand, and the albumen and colliquament being entirely consumed, all the membranes, except that which surrounds the vitellus, are dried up and disappear; the membrana vitelli, on the contrary, along with the yelk, is retracted into the peritoneum of the chick and included in the abdomen. Of the membranes two are common to the whole egg, which they surround immediately under the shell; the rest belong, one to the albumen, one to the yelk, one to the colliquament; but all still conduce to the preservation and separation of the parts they surround. The outer of the two common membranes which adheres to the shell is the firmer, that it may take no injury from the shell; the inner one again is smooth and soft, that it may not hurt the fluids; in the same way, therefore, as the meninges of the brain protect it from the roughness of the superincumbent skull. The internal membranes, as I have said, include and keep separate their peculiar fluids, whence they are extremely thin, pellucid, and easily torn.
Fabricius ascribes great eminence and dignity to the chalazæ, regarding them as the parts whence the chick is formed; he, however, leaves the spot or cicatricula connected with the membrana vitelli without any office whatsoever, looking on it merely as the remains of the peduncle whence the vitellus was detached from the vitellarium in the superior uterus of the hen. In his view the vitellus formerly obtained its nourishment either by this peduncle or the vessels passing through it; but when detached, and no longer nourished by the hen, a simple trace of the former connexion and important function alone remains.
I however am of opinion that the uses of the chalazæ are no other than those I have assigned them, namely, that they serve as poles to the microcosm of the egg, and are the association of all the membranes convoluted and twisted together, by which not only are the several fluids kept in their places, but also in their distinct relative positions. But I have absolute assurance that the spot or cicatricula in question is of the very highest importance; it is the part in which the calor insitus nestles; where the first spark of the vital principle is kindled; for the sake of which, in a word, the whole of the rest of the fluids and all the membranes of the egg are contrived. But this has been already insisted on above.
Formerly, indeed, I did think with Fabricius that this cicatricula was the remains or trace of the detached peduncle; but I afterwards learned by more accurate observation that this was not the case; that the peduncle, by which the vitellus hangs, was infixed in no such limited space as we find it in apples and plums, and in such a way as would have given rise to a scar on its separation. This peduncle, in short, expands like a tube from the ovary on towards the vitellus, the horizon of which it embraces in a bipartite semicircle, not otherwise than the tunica conjunctiva embraces the eye; and this in suchwise that the superior part of the vitellus, or the hemisphere which regards the ovary, is almost free from any contact or cohesion with the peduncle, in the superior part of the cup or hollow of which nevertheless, but somewhat to the side, the spot or cicatricula in question is placed. The peduncles becoming detached from the vitelli can therefore in no way be said to leave any trace of their attachments behind them. Of the great importance of this spot in generation I have already spoken in the historical portion of my work.
But I have still, always following my old teacher Fabricius as my guide on the way, to treat of the uses of the cavity in the blunt end of the egg.
Fabricius enumerates various conveniences arising from this cavity, according to its dimensions. I shall be brief on the subject: it contains air, and is therefore useful in the ventilation of the egg, assisting the perspiration, refrigeration, and respiration, and finally the chirping of the chick. Whence this cavity, small at first, is larger by and by, and at last becomes of great size, as the several offices mentioned come into play.
Thus far have we spoken of the generation of the egg and chick, and of the uses of the several parts of the egg; and to the type exhibited we have referred the mode of generation of oviparous animals in general. We have still to speak of the generation of viviparous animals, in doing which we shall as before refer all to a single familiarly known species.