Charles R. Bardeen 263 
5. Development and innervation of the muscles supplied 
OOP MAA TRAE GOKU WEPRDEs oo bc op obo aoduonGeoos 366 
INES Cre Ohl=AKormbtcl IRM; cdo caccadocacobopoupDocanc 366 
NoHo HOO INNS Gao ecosocadgandoonccoomeazooasans 367 
IMD Cove MOR VDIOKENEY loreeNAISA Sts ob ood ooo Undo ODDO Oboe dao oR 367 
Comparative anatomy of the intrinsic plantar 
boaHU ISTO) eit eee ote ciao Craig o ltd Gobo COCO occa ome 368 
6. Muscle branches of the plantar nerves............... 369 
ML Sinise hay Eail CeopKolhwispouiss asm ceoadacécaqdooeocooongdboduOdd 369 
D. PERINEAL MUSCULATURE AND NERVES OF THE PUDIC GROUP...............; 374 
Os IHR OV OME CLVOCMMEDE bacconusasccueoosaasboncbeuoOmoD 374 
Os INGRRE CUPUHHOM Uh HIE CHWs occ aaoanenecuckacanenoooedDeE 380 
In a previous article in this journal (Bardeen and Lewis, o1), an 
outline was given of the early development of the limbs, body-wall and 
back in the human embryo. Lewis subsequently, 01, gave a more detailed 
account of the development of the arm, and I have recently, 05, described 
at some length the development of the spine and of the skeleton of the 
leg. The purpose of the following paper is a more detailed account of 
the development of the nerves and musculature of the leg and of the 
neighboring regions of the trunk and a consideration of the relation of 
developmental conditions to variations found in the adult. The em- 
bryological studies have been based chiefly on embryos belonging to the 
collection of Professor Mall of the Johns Hopkins University, who kindly 
placed them at my disposal. The statistical studies of nerve variation 
are based upon charts drawn from specimens in the dissecting rooms of 
the Johns Hopkins University and at the University of Wisconsin. 
A. OUTLINE OF THE DEVELOPMENT OF THE MUSCLES 
AND NERVES OF THE INFERIOR EXTREMITY. 
I. GENERAL FEATURES. 
For a description of the development of the external form of the limbs 
and of the chief features which characterize the earlier stages in the 
internal differentiation, reference may be made to the three papers men- 
tioned above. The posterior limb-bud is first seen as a massing of the 
mesenchyme at the posterior extremity of the Wolffian ridge, usually 
opposite the 21st to the 26th spinal segments. This mesenchyme arises 
in part from the axial mesenchyme, in part possibly from the somato- 
pleure. There is no good evidence that in the mammals the myotomes 
contribute directly to it. On the contrary the myotomes are sharply 
marked off by a limiting membrane from the mesenchyme of the limb- 
