OVIPAROUS ANIMALS. 419 
quires a determinate temperature to excite the germ to ac- 
tion; the nourishment is obtained from the glaire and the 
yolk by means of umbilical vessels, and the process of aéra- 
tion is effected through the pores of the shell, and the walls 
of the folliculus aéris, or by the intervention of the common 
membrane or glaire*: The hatching is accomplished at 
different periods, and by the use of different means. 


* The following description of the appearances of the incubated egg of 
the common hen, may not be unacceptable to the reader: ‘¢ A small shin- 
ing 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 ci- 
catricula, but very near that part on the yolk-bag. This may be said to 
appear beforehand, 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, resembling a gelatinous filament with 
large extremities, very closely surrounded by the amnion, which at first can 
scarcely be distinguished from it. 
‘© About this time the halones enlarge their circles ; but they soon after 
disappear entirely, as well as the cicatricula. “ 
«¢ The first appearance of red blood is discerned on the surface of the yolk- 
bag, towards the end of the second day. A series of points is observed, 
which form grooves, and these closing, constitute vessels, the trunks of 
which become connected to the chick. The vascular surface itself is called 
JSigura venosa, or area vasculosa; and the vessel by which its margin is de- 
fined, vena terminalis, The trunk of all the veins joins the venz porte ; 
while the arteries which ramify on the yolk-bag arise from the mesenteric 
artery of the chick. 
“* On the commencement of the third day, the newly formed heart is 
discerned by means of its triple pulsation, and constitutes a threefold punc- 
tum saliens. Some parts of the incubated chicken are destined to undergo 
successive alterations in their form, and this holds good of the heart in par- 
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, which is properly the right, is then the common auricle, the other is 
the common ventricle, but afterwards the left; and the third is the dilated 
part of the aorta. 
Dia 2 
