THE PHYLOGENY OF THE CEUEAL FLEXOES. 



BY 



J. PLAYPAIR McMURRICH. 



From the Anatomical Laboratory of the University of Michigan. 



With 14 Text Figuees. 



In an earlier paper (1903) I presented the results of a comparative 

 study of the flexor muscles of the antibrachial region and showed that it 

 was possible to trace step by step the changes by which the arrangement 

 occurring in the Urodelous amphibia was converted into that charac- 

 teristic of the mammalia. In the amphibia the muscles in question 

 possess a definite arrangement in layers and it was shown that these 

 layers have a fundamental significance, since, notwithstanding the almost 

 innumerable modifications and differentiations which they present in 

 higher forms and the apparently enormous differences which exist be- 

 tween the amphibian and mammalian forearm musculatures, yet the 

 layers could be recognized throughout and consequently afforded a basis 

 for the reconstruction of the phylogeny of the mammalian muscles. 



It became of interest, consequently, to ascertain whether a compara- 

 tive study of the crural flexors would reveal a similar fundamental 

 arrangement in layers and so afford a basis for their phylogenetic re- 

 construction, and, if so, an opportunity for a satisfactory consideration 

 of the much-discussed question of the serial homology of the arm and 

 leg musculature. In the following pages the results of such a study 

 are recorded in so far as they bear upon the first of the two problems men- 

 tioned, namely, the phylogeny of the crural flexors. The question of 

 the serial homology of the arm and leg musculature I hope to discuss 

 later in connection with some other general questions relating to the 

 morphology of the vertebrate limb. 



The methods and forms employed in the present study were essen- 

 tially the same as those made use of in the investigation of the arm 

 muscles. The arrangement and relations of the muscles were studied 

 in serial transverse sections and the forms employed were Amhlystoma 

 tigrinum as a representative of the Urodele amphibia, Scincus sp? as 

 American Journal of Anatomy. — Vol. IV. 



