100 



THE EARLY DEVELOPMENT OF MAMMALS. 



once branches to the right and left. Each branch forms five vessels, the so-called 

 aortic arches (Figs. 55, 92, and 93). The first arch is the first formed, the other 

 four are formed in succession behind it. The arches show a constant relation to 

 the pharyngeal gill-pouches, there being one pouch between every two arches. The 

 arches pass dorsalward around the broad pharynx (Fig. 28), and those of each 

 side become united by a single dorsal longitudinal vessel (Fig. 56, Ao.D}, the 

 dorsal or descending aorta. The two dorsal aortae pass caudad until they meet 

 and unite in the median line to form the main aorta.. (Compare Figs. 196, Ao.S; 



Ao.D 



. IV 



II 



XEO 



Coe. 



Ao.M 



Car. ex 



FIG. 55. ANTERIOR WALL OF THE PHAR- FIG. 56. PIG EMBRYO OF 6.0 MM. SERIES 9. AORTIC ARCHES OF 



YNX OF A HUMAN EMBRYO OF 3 . 2 MM. LEFT SIDE. FROM A WAX RECONSTRUCTION BY L. M. FERGUSON. 



1-5, Gill-arches; the arches are separated I, II, III, TV, V, Aortic arches. Car. ex, External carotid. Ao, Cardiac 



from one another by the entodermal aorta. Ao. D, Dorsal aorta. Ao.M Median aorta. 



and thecorrespondingectodermal gill- 

 pouches; the aortic arches are drawn 



in dotted lines and arise from the 



median cardiac aorta. M, Mouth. 



Oe, (Esophagus. Cce, Coelom. X 50 



diams. (After W. His.) 



197, Ao, and 198,^0.) In a pig of 7.8 mm. the five aortic arches can be still 

 traced, but the first arch has begun to disappear, and the condition illustrated in 

 figure 169 is established. Its ventral portion, /, persists, however, and together with 

 its own vascular prolongations into the lower Jaw gives rise to the external 

 carotid (Fig. 101, car. ex). The descending aorta on the dorsal side between the 

 tops of the first and second arches also persists and is prolonged into the head to 

 constitute the internal carotid (Fig. 172, car.i). Presently the second arch also 

 disappears, and both carotids are, as it were, thereby lengthened. This is the 

 condition which we find in our embryo of 12 mm. (Fig. 172). The third, fourth, 

 and fifth arches are still present. From the base of the third arch runs forward 

 the external carotid, and from the summit of the third arch runs forward the 



