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AVES BIRDS. 



chial vessels of the corresponding 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 mean time 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. 



At this period three fissures are perceptible between the bran- 

 chial 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. 



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 cel- 

 lular 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 recognisable ; 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. 



The second arch then becomes diminished in size, insomuch that 

 the third and fourth receive the greater part of the blood ; while in 

 the meanwhile a fifth arch makes its appearance behind the fourth, 

 so that in this way there are still four permeable arches. 



While these changes are going on in the vascular canals, the 

 first branchial fissure gradually closes ; and, to make up for this,' a 

 new one is formed between the arch which originally was the fourth, 

 and that last developed. 



At the commencement of the fifth day there are consequently 

 again four vascular arches and three branchial fissures on each side ; 

 but not the same as those of the third day, since one arch and one 

 fissure have disappeared, and have been replaced by similar parts. 

 During the fifth day the vascular arch, which at first was the 

 second, is obliterated, and the two succeeding ones become increased 

 in size ; but at the end of the fifth day all the branchial fissures are 

 effaced, being filled up with cellular tissue, and no trace of them is 



