COMPARATIVE ANATOMY. 
tlie cicatiicula itself, and of the chalazce, is 
not yet ascertained. 
We now proceed to notice the wonder- 
ful successive changes which go on during 
tlie incubation of tlie egg, and the meta- 
morphoses which are observed both in the 
general form of the chick and in particular 
viscera. The periods of these changes will 
be set down from the hen, as affording the 
most familiar example. It will be best to 
give, first, a cursory chronological view of 
the whole process, and then to make a few 
remarks on some of the most importairt 
parts of the subject. 
A small shining spot of an elongated form, 
with rounded extremities, but narrowest in 
the middle, is perceived at the end of the 
first day, not in nor upon the cicatiicula, 
but very near that part on the yolk-bag 
(nidus pulli ; colliquaraentum ; areola pellu- 
cida). This may be said to appear before- 
hand as the abode of the chick which is to 
follow. 
No trace of the latter can be discerned 
before the beginning of the second day ; 
and then it has an incurvated form, resem- 
bling a gelatinous filament with large ex- 
tremities, very closely surrounded by the 
amnion, which at first can scarcely be dis- 
tinguished from it. 
About this time the halones enlarge their 
circles, but they soon after disappear en- 
tirely, as well as the cicatiicula. 
The first appearance of red blood is dis- 
cerfied on the surface of the yolk-bag to- 
wards the end of the second day. A series 
of points is observed which form grooves ; 
and these, closing, constituf e vessels, the 
trunks of which become connected to the 
chick. The vascular surface itself is called 
hgura venosa, or ai’ea vasculosa ; and the 
vessel, by which its margin is defined, vena 
terminalis. The trunk of all the veins Joins 
the vena portae ; while the arteries, which 
ramify on the yolk-bag, arise from the me- 
senteiic artery of the chick. 
On the commencement of the third day 
the newly-formed heart (the primaiy organ 
of the circulating process which now com- 
mences) is discerned by means of its triple 
pulsation, and constitutes a threefold punc- 
tum salieus. Some parts of the incubated 
chicken are destined to undergo successive 
alterations in their form ; and this holds 
good of.the heart in pai'ticular. In its first 
formation it resembles a tortuous canal, 
and consists of three dilatations lying close 
together, and arranged in a triangle. One 
of these, whioli is properly the right, is then 
the common auricle; the otiier is the only 
ventricle, but afterwards the left ; and the 
third is the dilated part of the aorta (biilbus 
aortae). 
About the same time, the spine, which 
W'as originally extended in a straight line, 
becomes incurvated ; and the distinction of 
the vertebra*, is very plain. The eyes may 
be distinguished by tlieir black pigment, 
and comparatively immense size ; and they 
are afterwards remarkable in consequence 
of a peculiar slit in tlie lower part of tlie 
iris. 
From the fourth day, when the chicken 
has attained the length of four lines, and 
its most important abdominal viscera, as the 
stomach, intestines, and liver, are visible, 
(the gall bladder, however, does not ap- 
pear till the sixth day), a vascular mem- 
brane (chorion, or membrana iimbilicalis) 
begins to form about the navel, and in- 
creases in the following days with such ra- 
pidity, that it covers nearly the whole inner 
surface of the shell within the membrana 
albuminis, during the latter half of incuba- 
tion. This seems to supply the place of the 
lungs, and to carry on the respiratory pro- 
cess instead of those organs. The lungs 
themselves begin indeed to be formed on 
the fifth day ; but, as in the foetus of tlie 
mammalia, they must be quite incapable of 
peiforming their functions wliile tlie chick 
is contained in the amnion. 
Voluntary motion is first observed on the 
sixth day ; when the chick is about seven 
lines in length. 
Ossification commences on the ninth 
day, when the ossific juice is first secreted, 
and hardened into bony points (puncta 
ossificationis.) 
These form the rudiments of the bony 
ring of the sclerotica, which resembles at 
that time a circular row of the most delicate 
pearls. 
At the same period, the marks of the 
elegant yellow vessels (vasa vitelli liitea) on 
the yolk-bag, begin to be visible. 
On the fourteenth day, the feathers ap- 
pear ; and tlie animal is now able to open 
its mouth for air, if Taken out ot the egg. 
On the nineteenth day it is able to ut- 
ter sounds ; and on the twenty first to 
break through its prison, and commence a 
second life. 
We shall conclude with one or two re- 
marks on tliose very singular membranes, 
the yolk-bag and chorion, which are so 
essential to the life and preservation of the 
animal. 
