496 FRANK REAGAN 



111 a dicephalous lamb, three or four weeks after its birth, a 

 vessel suggesting a fifth arch was found by Bishop ('08). It 

 was seen as a slender sinus, connecting the two component aortic 

 trunks. Since the more stable arches, as such, had either degen- 

 erated or become greatly modified, and since the vessel had no 

 connection with either aortic root, it affords rather uncertain 

 evidence of a fifth arch. 



In cat embryos, Coulter ('09) found spurs and sinuses which he 

 suggests as rudiments of a fifth vessel, but in no case did he find 

 a complete arch. 



If we are to be so exacting as to demand of this new arch, those 

 qualifications and characteristics common to other arches with 

 which we attempt to prove the vessel homologous, it will be 

 seen that the only typical fifth arches yet demonstrated, are 

 those of the eleven day rabbit of Zimmermann, and of the 5.5 

 mm. mole of Soulie and Bonne. These are described as typical 

 in that they were said to connect the aortae and occupied distinct 

 visceral arches. But the description of the vessel in the Zim- 

 mermann embryo was unaccompanied by figures; furthermore, 

 the existence of the vessel in the rabbit has been doubted by Lewis 

 (assent to Lewis's view has been given by Coulter) . Also, from the 

 description of the 5.5 mm. mole of Soulie and Bonne, the ventral 

 aorta is considered as extending to a point 30^ vertically above 

 the lumen of the fourth arch, so that the common trunk of the 

 fourth and fifth arches must have been interpreted as ventral 

 aorta. This interpretation is allowable, yet an arch arising more 

 ventrally would be more typical. From their reconstruction 

 'A,' the dorsal portions of the fifth vessel and the pulmonic arch 

 seem sufficiently intimate to justify their being regarded as entering 

 the dorsal aorta in common. The supposition is in harmony with 

 the view of Coulter in which he, reviewing the results of Soulie 

 and Bonne, stated that their fifth arches emptied "in every case 

 into the dorsal aorta in common with the pulmonic arch." At 

 any rate, this intimacy renders the arch less typical. From the 

 foregoing consideration, a demonstration of an arch more typical 

 than those just mentioned seems desirable, even though it be 

 granted with Soulie and Bonne that they may have successfully 

 demonstrated their " phylogenetic souvenir." 



