370 Remarks on the Physiology of the Ege. 
logists; I shall confine myself, therefore, to what I consi- 
der exclusively original. 
The principal use of the albuminous portion of the egg 
is doubtless to afford materials for the growth, and nourish- 
ment for the support, of the ovular embryon: such how- 
ever does not appear to be the on/y purpose for which it 1s 
desigued. Nowhere does Nature display more anxiety 
for the preservation of her offspring, or more wisdom to 
obtam her objects, than in her provisions to ensure an 
equable temperature to the feedus in ovo: a condition which 
is so essential to the evolution of the animal, that the smallest 
deviation overthrows the nice balance between the different 
actions that are to mature it, and produces fatal effects. 
The allmmen then [consider as a great defence against such 
an evil. The chalaxa, by retaining the cicatricula at the 
source of heat, obviates the mischief that would accrue 
from constant change of position; but the aélumen, being 
a most feeble conductor of caloric, retards the escape of 
heat, prevents any sudden transition of temperature, and 
thus averts the fatal chills which the occasional migrations 
of the parent might induce. As an illustration of the use 
and importance of such a structure, I may observe, that 
those fish which retain their vitality a considerable time 
after their removal from the water, as eels and tench, have 
the power of secreting a slimy and viscid fluid, with which 
they envelop their bodies. Is it not extremely probable 
that this matter, by acting like the albumen of the egg, and 
preventing evaporation from the surface of the animal, and 
the consequent change of temperature, may be the prin- 
cipal cause of this tenacity of life? 
It must however be remarked, that deviations of tem- 
perature are injurious and fatal in proportion only to the 
degree of vital energy which the ovular embryon possesses : 
hence germs of inferior vitality not only suffer the vicissi- 
tudes of heat and cold with impunity, but are developed by 
a less defined temperature. We therefore perceive, as we 
descend the scale of oviparous heings, that those peculiar 
provisions which the eggs of perfect animals possess, for 
the regulation of their temperature, cease to be essential, 
and therefore disappear. 
The part of the egg to which I next beg to direct your 
attention is the folliculus aéris, or air-bag, placed at its ob- 
luse extremity; the nature of this follicle excited in mé 
considerable inierest, as I found that it had not been so 
fully investigated as its importance scemed to demand. * 
The external shell, and the internal miembrane by which 
‘ ely ut 
