to the small Intestines of Birds. 259 
although I do not perceive in what manner this peculiarity of 
structure becomes useful to them. 
The appendix in most birds retains the marks of its origin ; 
thus the remnant of the yolk bag is commonly found attached 
to its free extremity, with which it still preserves a communi- 
cation. This occurs particularly in the passerine tribe, and 
amongst the accipitres. In the nightingale the duct is so short, 
that it is scarcely visible ; but the yolk bag remains during 
life about the size of a small pea, adhering to the intestine 
with which it communicates. 
The preceding facts furnish a curious example of the oeco- 
nomy of nature, in adapting an organ of foetal life to the 
exercise of a particular function in the full grown bird. 
They likewise afford a proof that the yolk bag and intes- 
tine not only have communicated, but that they were originally 
continuous structures, a fact which has been lately denied by 
Leveille'. 
At a future period, I hope to lay before the Society, some 
observations upon the functions of the different parts of the 
egg during incubation, from which it will be found, that a 
communication between the yolk bag and intestines exists as 
a matter of necessity. 
LI 2 
