268 Dr Allen Thomson 07i the Vascular Si/stem 



into which the blood is still propelled from the bulb of the aorta 

 through the communicating vessel of the second arch (Fig. 30). 

 While the first pair of vascular arches is obliterated, a fifth 

 pair is formed behind the four which previously existed, pro- 

 ceeding in the same manner from the ascending to the descending 

 aorta. At the same time, the first branchial aperture, situated 

 between the first and second vascular arches, is gradually closed, 

 and a fourth appears between the fourth and fifth arches. Thus 

 there exist in the neck of the embryo of the chick five vascular 

 arches and four branchial clefts, corresponding with the appear- 

 ances in the neck of the salamander and fish, and the analogy 

 between these animals and the foetal chick before referred to be- 

 comes apparent (Fig. 20, m. o). Rathke has observed, that 

 the distribution of the arteries, as well as the structure of the 

 branchial hoops, in the foetus of the Blennius vivip., resemble 

 almost exactly those in the chick on the fourth and fifth days. 

 Baer remarks, that he has never seen more than four vascular 

 arches co-existent in the embryo of the chick. In the embryo 

 of the duck of four days and a half, I have observed four aper- 

 tures co-existing, and four vascular arches very apparent ; from 

 which it appears probable that a fifth also existed, though not 

 easily seen from the small quantity of blood it contained. 



On the fourth day, the second arch also becomes less, and on 

 the fifth day is wholly obliterated, while the third and fourth 

 now become stronger. Towards the end of the fifth day (See 

 Fig. 22, o), the three remaining clefts on each side of the pha- 

 rynx become gradually less perceptible, and are soon closed by 

 the union of the integuments on each side of the cleft. The an- 

 terior clefts remain open longer than the posterior, and while they 

 are closed on the outside, they are still visible on the inside of 

 the pharynx, in the form of small cavities opening into it (Fig. 

 23). The third arch, now the most anterior of those remaining, 

 forms thie brachial arteries. The vessels of the anterior extre- 

 mities spring from the place where the third vascular arch joins 

 the root of the aorta (Fig. 30, w). They may be perceived with 

 ease on the eighth day, and after this period, the branch (w) by 

 which the thiud arch joins the fourth in the root of the aorta 

 shrinks and gradually becomes smaller, till it entirely disappears 

 before the 13th or 14th day ; at which time the whole of the blood 



