400 Dr. Prout’s experiments on the changes, & c . 
to conceive how the earth in question can be introduced into 
the economy of the chick from this source, particularly during 
the last week of incubation, when a very large portion of the 
membranes are actually separated from the shell. Secondly, 
both the albumen and yelk contain, at the end of incuba- 
tion, a considerable proportion of earthy matter (the yelk 
apparently more than it did originally) ; why is this not 
appropriated, in preference to that existing in the shell ? 
In opposition to these arguments it will be doubtless stated, 
that the shell of the egg becomes brittle at the end of incu- 
bation, and appears to undergo, during that process, some 
other changes not at present understood. To which it may be 
answered, that this brittleness has been attributed to the sepa- 
ration of the membrana putaminis, and the exsiccation of the 
parts by so long an exposure to the heat necessary to the 
process of incubation ; and in this manner all the known 
changes produced on the shell by incubation may perhaps be 
satisfactorily accounted for. Until, therefore, it be demon- 
strated that some other changes take place in the shell, I con- 
fess this argument does not seem to me to have much weight. 
I by no means wish however to be understood to assert, that 
the earth is not derived from the shell ; because, in this case, 
the only alternative left me is to assert that it is formed by 
transmutation from other matter ; an assertion, which I con- 
fess myself not bold enough to make in the present state of 
our knowledge, however strongly I may be inclined to be- 
lieve that, within certain limits, this power is to be ranked 
among the capabilities of the vital energies. 
