92 CHANGES DURING INCUBATION. 



means of its triple pulsation, and constitutes 

 a triple or threefold punctum saliens. Some 

 parts of the chick are destined to undergo 

 successive alterations in their form, and this 

 holds good of the heart in particular. In 

 its first formation it resembles a tortuous ca- 

 nal, 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 afterward the left ; 

 and the third is the dilated part of the aorta, 

 or main bloodvessel of the body. About 

 the same time, the spine, which was origi- 

 nally extended in a straight line, becomes 

 curved, and the joints may be easily seen. 

 The eyes may be distinguished, looking 

 black and large. 



From the fourth day, when the stomach, 

 intestines, and liver are visible, a membrane, 

 full of bloodvessels (chorion),* begins to form 



* To the anatomist the appearance of this membrane is very 

 interesting. On its surface we see bright red vessels, carrying 

 blood with air to the chick, and dark, brownish-red vessels taking 

 the carbonated blood away. It is, in fact, a temporary substitute 

 for lungs. The yolk membrane is also CDnnected, but in a two- 



