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 aera- 

 tion is effected through the pores of the shell, and the walls 

 of the folliculus aeris, 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 

 figura venosa, or area vasculosa ; and the vessel by which its margin is de- 

 fined, vena terminalis. The trunk of all the veins joins the venae portae ; 

 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- 

 tual 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. 



