EXERCISE THE TWENTY-FOURTH.
Of twin-bearing eggs.
Twin-bearing eggs are such as produce twin chickens, and according to Aristotle,[196] “are possessed of two yelks, which, in some are separated by a layer of thin albumen, that they may less encroach on one another; in others, however, there is nothing of the sort, and then the two yelks are in contact.”
I have frequently seen twin eggs, each of the yelks in which was surrounded by an albumen, with common and proper membranes surrounding them. I have also met with eggs having two yelks connate, as it were, both of which were embraced by a single and common albumen.
“Some fowls” says Aristotle,[197] “always produce twins, in which the particulars relating to the yelk that have been stated are clearly perceived. A certain fowl laid within two of twenty eggs, all of which, except those that were unprolific, produced twins. Of the twins, however, one was always larger, the other smaller, and the smaller chick was frequently deformed in addition.”
With us twin eggs are occasionally produced, and twin chicks too, although very rarely, are engendered. I have never myself, however, seen both of these chicks live and thrive; one of them either died within the egg or at the time of the exclusion. And this the words of Aristotle prepare us to expect, when he says “one of the two is larger, the other smaller;” this is as much as to say that one of them is stronger and of greater age, the other weaker and less prepared for quitting the shell: my own opinion therefore is, that the two yelks are of different origins and maturity. It is therefore scarcely possible but that the stronger and more advanced chick, if the egg be broken and it emerge into the light, will cause the blight and abortion of the other. But if the stronger bird do not chip the shell, he himself is threatened with a present danger, viz. want of air. At the exclusion from the shell, consequently, certain death hangs over one or other, if not over both.
Fabricius either not observing the above words of Aristotle, or neglecting them, says: “If an egg have now and then two yelks, it engenders a chick having four legs or wings, and two heads—a monster, in short; never two chicks distinct from one another, and that can be spoken of as a pair; there is but one trunk, to which are appended two heads, &c.”
Whence we may infer that he himself had never seen nor heard from credible persons that such eggs produce two pullets, and therefore that he agrees with me in regarding such eggs as rare, and in holding that they never produce two chicks both alike capable of living.
I am surprised nevertheless that, with the authority of Aristotle before him, he should have said that “two chicks, distinct and separate, are never produced from such eggs,” but always a monster; the rather as he thinks that the embryo is engendered from the chalazæ as from the appropriate matter, and he could not but see that there are four chalazæ in every twin-egg.
I should rather imagine that when two vitelli are included by the same albumen in a twin-egg, and are so intimately associated that their cicatriculæ, when they are resolved together, constitute a single eye or colliquament, may engender a monstrous embryo with four feet, two heads, &c., because I see nothing to hinder this; and such a production do I conceive to have been engendered by the egg of which Fabricius speaks.
But where two yelks have existed separately, parted by their several membranes, and furnished with chalazæ, albumens, and all else requisite to the generation of the chick, I hold that we must conclude, with Aristotle, that such an egg, as it has all the parts of two eggs except the shell, so does it also possess the faculty or faculties of as many; and unless it be a wind or barren egg, that it will for the most part produce two embryos, and but rarely a single monstrous individual.