424 



ternal carotid is now present, and the third arch and anterior pro- 

 longation of the dorsal aorta indicate the course of the internal carotid. 

 The decrease in size of the dorsal aorta between the third and fourth 

 arches foreshadows the disappearance of that portion of the aortic root. 



The vessel described as a fifth aortic arch in this embryo, arises 

 at about the same stage as in other mammals as described by 

 Zimmermann and Tandler. The presence of an extra entodermal 

 pouch in this region, and the distinct character of the vessel, will not 

 admit of its being interpreted as a part of the fourth arch, or of the 

 dorsal aorta, and furnish good evidence toward the acceptance of this 

 artery as a fifth aortic arch. 



Embryo of the twenty-second day. The chief points to 

 be noted here, as shown in Fig. 13, are the wide dorsal separation 

 of the third and fourth arches with a very slender portion of the 

 aortic roots connecting them; the breaking down of the fifth arch, 

 and the complete separation of the bases of the systemic and pulmonic 

 vessels. 



A short common arotid {C.Car.)^ is present at the base of the 

 third arch, and from it spring the external and internal carotids. The 

 fourth arch is still a large vessel upon either side, but that of the 

 left side is larger than that of the right. Two entodermal pouches 

 exist between the fourth and sixth aortic arches in this embryo. 



Embryo of a later stage (twenty-third? day). Fig. 14 

 represents a reconstruction of the aortic arches on the right side in 

 an embryo of possibly the twenty-third day of development. 



The aortic arches are situated further back than in preceding 

 stages. The fourth aortic arches from either side unite at the point X. 

 It will be seen, therefore, that the common carotid stem now arises 

 from the upper portion of the united arches. That portion of the 

 dorsal aorta between the third and fourth aortic arches has become 

 greatly reduced, and has lost its connection with the third arch. The 

 diameter of the right fourth arch is now less than one-half that of 

 the left side. The lumen of the right sixth arch is greatly reduced in 

 size. It walls are much thickened and it is doubtful if it will now 

 admit of the free passage of blood. It unites with the left sixth arch 

 above the now single pulmonary artery. 



It should be said that the history of the subclavians, carotids, 

 and vertebral arteries has been omitted from the above account, but 

 is considered in the complete paper of which this is a partial abstract. 



