76 Dr. Trout's Experiments , fyc. 
certaining the exact quantity of lime any particular shell origi- 
ginally contains. There are, however, very strong reasons for 
believing that the earthy matter is not derived from the shell. 
In the first place, the membrana putaminis never becomes vas- 
cular, and seems analogous to the epidermis ; hence the lime of 
the shell, which is exterior to this membrane, is generally consi- 
dered by physiologists as extra-vascular *. It is, therefore, ex- 
tremely difficult to conceive how the earth in question can be 
introduced into the economy of the chick from this source, par- 
ticularly during the last week of ineubation, when a very large 
portion of the membranes are actually separated from the shell. 
Secondly, Both the albumen and yolk contain, at the end of in- 
cubation, a considerable proportion of earthy matter (the yolk 
apparently more than it did originally) ; why is this not appro- 
priated, in preference to that existing in the shell ? In opposi- 
tion to these arguments, it will be doubtless stated, that the shell 
of the egg becomes brittle at the end of incubation, and appears 
to undergo, during that process, some other changes not at pre- 
sent understood. To which it may be answered, that this brit- 
tleness has been attributed to the separation of the membrana 
putaminis, and the exsiccation of the parts by so long an expo- 
sure to the heat necessary to the process of incubation ; and in 
this manner, all the known changes produced on the shell by in- 
cubation may perhaps be satisfactorily accounted for. Until, 
therefore, it be demonstrated that some other changes take place 
in the shell, I confess this argument does not seem to me to 
have much weight. I by no means wish, however, to be under- 
stood to assert, that the earth is not derived from the shell ; be- 
cause, in this case, the only alternative left me, is to assert that 
it is formed by transmutation from other matter, — an assertion 
which I confess myself not bold enough to make in the present 
state of our knowledge, however strongly I may be inclined to 
believe that, within certain limits, this power is to be ranked 
* among the capabilities of the vital energies. 
* See an Essay “ On the Connection between the Vascular and Extra-vascular 
Parts of Animals,” by Sir A. Carlyle. Thomson’s Annals, vol. vi. p. 174. 
