DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 243 



and in the entire absence of any rudiment of a gill or gill process 

 this can hardly be wondered at. The development of the vessels of 

 the hyoid arch in the frog appears to us to indicate clearly the former 

 ancestral presence of gills on the arch, while the very early stage at 

 which the vessels undergo retrograde changes suggests that this 

 ancestral condition was a very remote one. 



Concerning the mandibular arch, our conclusions are as follows : — 



At 4| mm. (Fig. 4) there are no vessels in the mandibular arch. 



At 5 mm. (Fig. 6) there is in the lower or ventral portion of the 

 arch, forming the floor of the anterior part of the mouth, a lacunar 

 space, V M, which is in communication with the lacunar spaces into 

 which the hyoidean vein, V Y, also opens. From the dorsal aorta a 

 small diverticulum, E M, arises, which we have no doubt is to be 

 referred to the mandibular arch. 



At 6| mm. (Fig. 7) the two factors previously present have grown 

 considerably, and have met and opened into each other. We now 

 have a diverticulum from the dorsal aorta, E M, of some length, which 

 runs down the mandibular arch and opens into the dorsal end of the 

 mandibular vein, V M. This communication, which is a very definite 

 and unmistakeable one, affords strong evidence in support of our 

 comparison of the lacunar vessel, V M, with the efferent lacunar vessel 

 of the hyoidean or branchial arches. 



The diverticulum from the aorta, EM, lies distinctly in front of 

 the hyomandibular cleft, and must beyond doubt be considered as 

 belonging to the mandibular arch. 



At 9 mm. (Fig. 11) there is no change of importance in the vessels 

 of the mandibular arch, except that the communication between the 

 lower end of the mandibular vein and the sinuses of the floor of the 

 mouth is narrower than before. 



At 12 mm. (Fig. 13) the mandibular artery, AY, has the course 

 and relations of the pharyngeal artery of the adult. We have studied 

 the intermediate stages, and have no doubt that the dorsal part of 

 this artery, from its origin from the aorta to its point of division, is 

 the same thing as the vessel, E M, in the 9 mm. stage. We have not, 

 however, determined with certainty the time and mode of formation 

 of the arter}' we have called posterior palatine (Fig. 13, AS), nor 

 the precise fate of the mandibular vein (Fig. 7, V M), into which the 

 mandibular artery opens at 9 mm. We therefore conclude that in 



