﻿Disappearance of Fifth Aortic Arch in Vertehrates. 167 



j XIII. Note on the Cause of Disappearance of the Fifth 

 •\ Aortic Arch in Air - hreathing Y ertehrates. By 

 J. Graham Kerr. 



,1 (Read 28th October 1907.) 



I It is well known that there are commonly developed in 

 |i the lower vertebrates a series of six pairs of aortic arches, 

 of which the last (VI.) gives off the pulmonary artery where 

 it is present. In the higher vertebrates, as is equally well 

 known, there occurs typically a series of five pairs of aortic 

 arches, of which the last gives rise to the pulmonary artery. 

 The discoveries of van Bemmelen and Zimmermann showed 

 that this curious discrepancy was more apparent than real 

 as they found, in various amniotic vertebrates, more or less 

 distinct vestiges of an aortic arch between the fourth and 

 the last one, showing the latter to be morphologically the 

 sixth aortic arch, as in the lower vertebrates. This raised 

 a question of some interest as to what is the cause of the 

 reduction and practical disappearance of the fifth aortic 

 arch in the Amniota. The purpose of this note is to draw 

 the attention of the Society to the fact that the probable 

 answer to this question is afforded by the conditions met 

 with amongst the Dipnoi. The adult relations of the parts 

 concerned were clearly described by Boas for Ceratodus and 

 Protopterus, and they may be illustrated by the condition 

 met with in a young Lepidosiren of stage 38. 



«• c. d, e. 



Fig. 1. — Outline drawings of transverse sections through conus arteriosus 

 of a young Lepidosiren of stage 38. 



The text-figure shows a series of transverse sections through 

 the conus arteriosus at different levels. Towards the hinder 

 end of the conus {a) there are seen projecting into the 



