Page:The Works of William Harvey (part 1 of 2).djvu/368

 2GH ON GENERATION.

EXERCISE THE TWENTY-FOURTH.

Of twin-bearing eggs.

Twin-bearing eggs are such as produce twin chickens, and according to Aristotle, 1 " 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 mem- branes 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, 2 " 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.

1 Hist. Anim. lib. vi, cap. 3. 2 Ibid.