178 The Phylogeny of the Forearm Flexors 
and a basis for its comparison with the flexor brevis of the foot, it would 
be necessary to discover what structures, if any, represented the sublimis 
in the lower vertebrates. Thus the investigation broadened to include 
a determination of the phylogeny of the entire flexor-pronator mass of 
the forearm and it is to the results of this portion of the problem that 
the present paper will be devoted. I hope to consider at some future 
date the muscles of the leg in a similar manner and so return to the 
question of the equivalency of the muscles in the two limbs. 
A few words are necessary regarding the forms studied and the meth- 
ods employed. My first intention was to approach the question from the 
embryological side, and to study the development of the forearm muscles 
in embryos of Amblystoma tigrinum, Anolis sagrai, the rabbit and 
rman. I soon discovered, however, that this method would not yield the 
desired results, for in the mammalian embryos the forearm muscles, 
when first distinctly recognizable, have practically the adult arrange- 
ment. The same result has been obtained by Lewis (1902) in his ad- 
mirable study of the development of the arm in man, and it would seem 
that there is a very extensive condensation in the ontogenetic develop- 
ment of the limb muscles in the mammalia. It is probable that the 
entire phylogenetic history of the forearm muscles of man, for instance, 
is condensed into the stages during which the muscles are represented 
by an undifferentiated mesodermic blastema and that, therefore, anoma- 
hes of reversion are referable to the possibilities, dependent on past his- 
tory, latent in this blastema. 
The embryological method being then excluded, it was necessary to 
have recourse to comparative anatomy. Careful dissection revealed 
much that was of importance, but far more valuable results were ob- 
tained from the study of serial sections. From these the topographic 
relations of the various muscles and their nerve supply could be deter- 
mined with certainty, and the pictures presented were so much more per- 
fect and striking that I finally relied on the sections rather than on dis- 
sections, employing the latter mainly for confirmation. 
As types of the urodelous amphibia I studied by both dissections and 
sections Amblystoma tigrinum and by sections only Plethodon erythro- 
notum. Of the reptilia I studied Phrynosoma cornutum, Liolepisma 
laterale, Callisaurus draconoides and Chrysemys picta, and of mammalia 
IT examined Didelphys virginiana (the material of which I owe to the 
kindness of Dr. C. F. W. McClure, of Princeton University), the cat, 
! For material of this form I am indebted to the kindness of my friend, Dr. Henry 
Orr, of Tulane University. 
