746 AVES. 



arches (c) ; so we have yet to trace how, as the lungs increase in size, 

 the circulatory apparatus becomes changed and the branchial organs 



obliterated. 



Fig. 374. 



Embryo about the fifth day of incubation. 



(2121.) On the third day of incubation there exist four vascular 

 arches (fig. 373, c) on each side, having a common origin from the bulb 

 (&), which obviously represents the bulbus arteriosus of Fishes and 

 Reptiles, before described ; these encircle the neck, and join on arriving 

 in the dorsal region to form the aorta, which commences by two roots, 

 each made up of the union of the four branchial vessels of the corre- 

 sponding side. The vascular arches are developed one after the other, 

 the most anterior being visible even on the second day; shortly, a 

 second appears behind the first, the former in the meantime becoming 

 considerably larger ; and at length the third and the fourth are formed, 

 the fourth being still very small at the commencement of the third day. 



(2122.) At this period three fissures are perceptible between the 

 branchial arches, and in front of the first pair is the first appearance of 

 the oral orifice, which, however, is not, properly speaking, the aperture 

 of the mouth, since at this epoch the jaws and buccal cavity are not as 

 yet formed, but, physiologically considered, it rather represents the 

 pharynx. 



(2123.) At the close of the third day this branchial apparatus is 

 already slightly changed : the branchial fissures are wider, and the fourth 

 vascular arch is become nearly as large as the others. On the fourth 

 day the first vascular arch is almost imperceptible, and that for two 

 reasons : in the first place, it becomes covered up with cellular tissue ; 

 and secondly, it is so much diminished in size towards the second half of 

 the fourth day, that it merely gives passage to a most slender stream of 

 nearly colourless blood. By the close of the fourth day it is no longer 

 recognizable, but, before its disappearance, it is seen to have given off 

 from its most convex point a vessel, which becomes the carotid artery ; 

 so that, when the arch itself is atrophied, that portion of it which was 

 connected with the bulb of the aorta becomes the trunk of the carotid. 



(2124.) The second arch then becomes diminished in size, insomuch 



