in tite Foetus of Verlehrated Animals. 257 



hoops of the foetus of the batrachia or of fishes, before the leaflets 

 of the gills are formed. 



The aorta, in the mean time, begins to divide itself into 



i/vessels which correspond with branchial vascular arches. At 

 the end of the second day, the aorta rising from the bulb be- 

 hind the pharyngeal portion of the intestine, runs forward along 

 the middle and lower part of this cavity, till it approaches the 

 opening of the mouth ; here it divides into two branches, which 

 separating from one another, proceed round the sides of the in- 

 testine close to the angle of the opening of the mouth, and jjoin 

 again near the vertebral column to form the descending aorta. 

 During the first half of the third day, a second pair of vascu ar 

 arches is formed behind the first, which encompasses the pharynx 

 in a similar manner; and towards the end of the third day, two 

 other pairs of vascular arches being formed, the anterior part of 

 the intestine is surrounded by four pairs of vascular arches, 

 rising successively from the aorta on the lower side, and joining 



, into the two roots of the aorta on the upper side of the intestine. 



*>'On the third day, according to Baer, the foremost of these pairs 

 of vessels which may be called branchial, the one first produced 

 is the largest, and makes the widest sweep ; the fourth^ or pos- 

 terior, is very small and scarcely perceptible. 



Each of these vascular arches, in winding round the side of the 

 pharynx, passes along one of the parts which correspond to the 

 branchial hoops ; so that each of the three clefts or apertures on 

 each side of the pharynx is situated between two of the vascular 

 arches. At the end of the third and beginning of the fourth 

 day, all the arches, and more especially the fourth, become 

 larger and fuller of blood, and, at the same time, the branchial 

 hoops become thicker, and the apertures between them wider ; 

 but in the course of this day the first vascular arch, having at- 

 tained its full size, soon begins to be less visible, both on account 

 of its own diminution or partial obliteration, and of the enlarge- 

 ment and increased opacity of the branchial hoop along which 

 it passes. Towards the end of the fourth day, this arch is 

 wholly obliterated, and no longer allows of the passage of blood 

 into the root of the descending aorta. A vessel proceeding to 

 the head and neighbouring parts, which afterwards becomes the 

 carotid, has, however, taken its origin from its most imtcrior part, 



