335 



phagei muscles of the adult may thus perhaps belong, one to each of the 

 two aborting clefts, the nerves that innervate them representing all 

 that is left of the nerves of the related arches. 



Regarding the arteries, v. Kupffer says (p. 69), in speaking 

 of his embryo IX: "von der in Fig. 61 gezeichneten medianen Ver- 

 einigung des vordersten Aortenbogens gehen kaudalwärts paarige Aorten 

 ab, die in Fig. 62 schräg durchschnitten über den Wülsten des Velum 

 zu sehen sind. Hinter der Hyomandibulartasche vereinen sich mit der 

 Einmündung des folgenden Aortenbogens die Aorten wieder über die 

 Mittellinie hinweg, und es setzt sich dann eine unpaarige Aorta fort." 

 The anterior, or prehyoidean, median section of the aorta here referred 

 to, is called by Müller (21) the "vertebralis impar capitis". 



The arteries in my 12 mm specimen do not exactly accord with 

 this statement of v. Kupffer for a somewhat younger embryo. In 

 my embryo, what I consider to be the two common carotids unite, 

 as they are followed backward, to form the single, median verte- 

 bralis impar capitis. While still dorsal to the velar folds this 

 artery separates into two parts, each of which turns sharply down- 

 ward anterior to the hyobranchial cleft of its side of the head. 

 Each artery, at the bend, sends a small branch backward, in 

 the direction of the main artery before the bend, and, uniting, 

 these two branches form a small median dorsal aorta. The main 

 artery, on each side, having reached a point ventral to the hyo- 

 branchial cleft, turns sharply backward ventral to that cleft and so 

 continues ventral to all the clefts until it reaches the truncus arteriosus. 

 At the point where it turns sharply backward ventral to the hyo- 

 branchial cleft, it takes up an artery coming from the anterior regions 

 of the head; and immediately anterior to each cleft it sends a short 

 but stout branch upward. These latter branches can only be traced 

 to the dorsal level of the related cleft, and certainly none of them 

 continue upward to join the dorsal aorta. The hyobranchial and 

 next posterior clefts do not, as already stated, open on the outer 

 surface. 



In the stage that I have next older than this so-called 12 mm 

 one, a 20 mm embryo, the vertebralis impar capitis separates into 

 two parts, as in the younger embryo, but each of these two parts, in- 

 stead of turning sharply downward and then sharply backward, curves 

 gradually downward and backward and so reaches the ventral aspect 

 of the first recognizable cleft, which cleft opens on the outer surface. 

 Immediately anterior to this cleft a branch is sent upward to a point 

 slightly above the level of the cleft, where it turns backward, and so 



