CHANGES DURING INCUBATION. 91 



pense of the white, the fluid part of which 

 it absorbs. No trace of the chick can be 

 discerned before the beginning of the sec- 

 ond day, when it has an incurvated form, 

 resembling a gelatinous body, with large 

 extremities, very closely surrounded by the 

 amnion, which at first can hardly be distin- 

 guished from it. About this time the ha- 

 lones enlarge their circles ; but they after- 

 ward disappear entirely, as well as the cica- 

 trices. 



The first appearance of red blood is dis- 

 cerned 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, constitute vessels, the 

 trunk of which becomes connected with the 

 chick. This is called the vascular surface, 

 and is bounded by what is called the termi- 

 nal vein. The trunk of all the veins joins 

 the vence portce, while the arteries, which 

 ramify on the yolk-bag, arise from the mes- 

 enteric artery of the chick. 



At the commencement of the third day 

 the newly-formed heart is discerned by 



