Benham. — Occurrenct of Tiro Unusual Blood-vcsseU in Hyla avirea. 31 



blood-vessels conform to a type in all tlie individuals of a species or even 

 genus, yet the mode of branching, the number of the branches, their size 

 and extent and distribution, are very rarely identical over a series of indi- 

 vidual specimens. So it is with other organs or systems of organs : some 

 of these " variations " are in the direction of loss of parts, or they may be 

 of additions of parts — new structures which suddenly appear without any 

 transition between them and the usual state of the organ. We call these 

 " mutations "" if they are hereditary, though in a large number of cases 

 it is impossible to determine whether this is so or not. But others of the 

 variations from the usual adult structure are due to the persistence of con- 

 ditions which are present in the embryo or in some lower member of the 

 group to which the species belongs, and which are usually lost during the 

 development, so as to be absent in the adult. Such persistent embryonic 

 structures are always of great interest. Two such cases were met with 

 amongst the adult frogs, Hyla aurea, dissected in my laboratory during this 

 session. Both of these conditions seem to be extremely rare, and, so far as 

 I have been able to consult the literature at my disposal, they appear to be 

 unique. 



1. Persistence of the Third Branchial Aortic Arch. 



On the 28th March one of my students called my attention to the 

 presence in the specimen she was dissecting of four arterial arches on each 

 side, in place of the normal three. The frogs had been injected so that 

 the students should be able to trace out the arterial system, and in the 



Fig. 1. — Arterial aortic arches ( X 2) showing persistent third larval arch, m, portion 



of petrohyoid muscle; I, small laryngeal arteiy. 

 Fig. 2. — Visceral veins in abnormal frog (from drawing by E. W. Hunt). The heart 



is turned forwards to show the sinus venosus and the hepatic veins. Iv, the 



unusual paired lateral (abdominal) vein; v\., the vesicular vein entering 



the hepatic portal. 



case under consideration the injection, performed by my assistant, Miss W. 

 Farnie, was particularly successful, so that even small branches were dis- 

 tended with the injection mass, such as the laryngeal and pharyngeal 

 arteries. The student had dissected out the arteries on the animal's right 

 side with great care, and only the internal carotid had been ruptured. The 

 other side she had scarcely touched when she directed my attention to its 

 condition. 



