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. 



