293 
fifth days of development. Its period of existence is much briefer 
than that of the other aortic arches and also it is subject to greater 
individual variation. Inasmuch as it can not be found in many series 
Fig. 4. Fig. 5. 
Fig. 3, 4, 5. Three sections through the region of the aortie arches of a chick 
embryo nearly five days old. >< about 48 diameters. 
of sections of the right age, it is probable that it is frequently enti- 
rely absent. 
Comments on the Condition of the fifth and sixth 
Aortic Arches in Mammals and other Vertebrates. The 
anatomical relationships of the fifth and sixth aortic arches having now 
