THE LATER DEVELOPMENT OF THE FROG 183 



In the fourth branchial arch, which lacks external gills, the 

 history is essentially modified only to the extent of the omission 

 of the vessels related to these structures. The vessels of this 

 arch appear considerably later than in the anterior arches. 



The development of the vessels of the mandibular and hyoid 

 arches seems to be quite variable among the different species 

 of Rana. The most consistent account is that of Marshall and 

 Bles, of R. temporaria. Here, in tadpoles of about 5 mm. a 

 lacunar vessel representing the aortic arch (the second of the 

 whole series) appears in the hyoid arch, and a small outgrowth 

 of the lateral dorsal aorta extends toward it, but never actually 

 joins it, disappearing about the time the mouth opens (Fig. 66, 

 A). At hatching a small outgrowth of the truncus arteriosus 

 may be seen extending into the lower end of the hyoid arch; 

 this has a very brief duration. At the same time the vestige 

 of the aortic arch has .divided into dorsal and ventral portions; 

 of these, the former soon disappears while the latter, now known 

 as the hyoidean vein, connects with a large vascular sinus in 

 the region of the oral sucker. 



The vessels of the mandibular arch appear shortly before 

 hatching; these are, a lacunar vessel in the lower part of the 

 arch, representing the original aortic arch, and a small out- 

 growth of the lateral dorsal aorta into the dorsal part. Soon 

 these unite and also join the hyoidean vein. After the mouth 

 opens, the outgrowth from the lateral dorsal aorta separates 

 from the other vessels and grows forward as the pharyngeal 

 artery, while the hyoidean vein disappears with the oral sucker 

 (Fig. 66, B). 



The continuations of the lateral dorsal aortse into the head 

 form the roots of the anterior or internal carotid arteries, whose 

 numerous branches supply the organs of the whole dorsal 

 part of the head; the internal carotids become connected by 

 two transverse commissural arteries passing anterior and post- 

 erior to the infundibulum (Fig. 66). The ventral part of the 

 head is supplied by the lingual or external carotid arteries; these 

 vessels appear, some time before the opening of the mouth, 

 as a pair of sinuses in the floor of the buccal cavity and 



