EMBRYOLOGY OF CRYPTOBRANCHUS 545 



most primitive of the dipnoi, and the amphibia, especially the 

 iirodeles, are numerous and important, and cannot be explained 

 as parallelisms." In this connection it is important to compare 

 the development of the circulatory system of Polypterus. From 

 the account given by Kerr ('07), it appears that in Potypterus 

 the vascular system, particularly because of the presence of only 

 a single pair of aortic arches, is decidedly less amphibian in 

 character than that of Ceratodus. 



In the present state of our knowledge it is impossible to reach 

 an unqualified decision of the question under consideration. 

 In weighing the evidence one should not forget that in numerous 

 'cases where anatomical and embryological evidence have come 

 into conflict in deciding questions of the phylogenetic relationships 

 of the larger groups of animals, it is the embryological evidence 

 that has had to give way, and that recent anatomical evidence 

 has to give place to paleontological. Whatever light may be 

 shed by future discoveries on the question of the derivation of 

 the amphibia from the crossopterygii or the dipnoi, it is clear 

 that the point of origin is not far from either stock; in otherwords, 

 that the three lines of descent have separated from a common 

 stem at no very great intervals. 



Concerning the immediate ancestry of the living amphibia 

 we have detailed evidence only in the case of one group, which 

 fortunately for our purpose is the caudata or urodela. According 

 to Moodie ('08 b) the urodeles are descended from the branchio- 

 sauria, a group of primitive extinct amphibia from the carbon- 

 iferous and Permian. These are small, short-tailed amphibians 

 with broad heads. The skull is slightly more complex than in 

 -the urodeles, and there is a dermal exoskeleton consisting of 

 rows of thin semi-cycloid scales, especially on the flanks and under 

 side of the body. External gills are present in the larvae. The 

 view that these forms are ancestral to the urodeles is based on 

 a detailed comparison of the structure of the skull, the structure 

 and form of the vertebrae and the ribs, the number of digits, 

 the arrangement of the phalangeal elements, the character of 

 the pectoral and pelvic girdles, the distribution of the lateral 

 line system, the structure and form of the long bones, and finally 



