in the Fixed Principles of the Egg during Incubation. 75 
rated in a striking manner by the present inquiry. Mr Hat- 
chett has also made the important and curious remark, that in 
the ova of those tribes of animals, the embryos of which have 
bones, there is a portion of oily matter ; and in those ova whose 
embryos consist entirely of soft parts, there is none. Hence it 
is concluded, that a certain portion of oil is necessary for the 
formation of bone. The present inquiry cannot be said to con- 
firm or invalidate this remark, for although in the earlier stages 
of incubation, before ossification has commenced, a portion of 
the oil of the yolk is appropriated to the purposes of the econo- 
my of the animal, yet by far the greater portion of it remains ; 
and some of it is even retained by the yolk till its final disap- 
pearance *. One great use of the yolk evidently is to furnish 
the phosphorus,, entering as phosphoric acid, into the skeleton 
of the animal ; but that the earthy portion of the bones is de- 
rived from the transmutation of the oil into lime, cannot, per- 
haps, be safely asserted in the present state of the inquiry. 
With respect to the earthy matter found in the skeleton of the 
chick when it quits the shell, I think I can venture to assert, 
after the most patient and attentive investigation, that it does not 
pre-exist in the recent egg ; certainly not, at least in any known 
state. The only possible sources, therefore, whence it can be 
derived, are from the shell, or transmutation from other princi- 
ples. Whether it be actually derived from the shell, cannot be 
determined by chemistry ; because, as we have seen, the shells 
of different eggs differ so much, that the application of averages 
is out of the question ; and we are of course precluded from as- 
Pliny makes the same remark. Ipsum animal ex albo liquore ovi corporatur. 
Cib'us in luteo est. Hist. Nat. x. 53. 
a I examined a chick on the eighteenth day after incubation. . The yolk was 
now feduced to less than 2 grs., but it was of its original yellow colour, and of 
course contained oily matter. When burnt, it left traces of phosphate of lime. 
Dr Macartney attempts to show that the yolk does not pass into the intestine 
through the ductus vitello intestinalis, but is taken up by absorption ; and an ar- 
gument he adduces in support of this opinion is, that the earthy matter is left 
behind in the yolk. In the present instance, however, the quantity of earthy 
matter was very minute : it had therefore disappeared, as well as the other prin- 
ciples of the yolk. When the chick is younger, the quantity of earthy matter is 
said to be much larger. Haller asserts that the yolk disappeared about the six- 
teenth day ; and Aristotle long ago remarked, that very little of it was left on the 
tenth day, after the chick had left the egg. 
