154 



The Development of the Arm in Man 



our next stage, embryo CLXIII, quite marked changes have taken place. 



Embryo CLXIII is 9 mm, in length, and about four and one-half 

 weeks old. 



In order to gain a clear conception of the form and various relations 

 of the structures in this embryo, I found it necessary to construct a 

 model, and in order to do so it was necessary to draw sharp lines about 

 the various structures when in reality there were no sharp limits. One 

 mass often shading off into another, while the central portion of each 

 was very distinct. Thus, in most places the skeletal core of the arm 



"eural process 



Fig. 6. Skeleton and nerves of the arm region in embryo CLXIII. 

 X 40 diameters. 



shades off into the surrounding premuscle tissue, or as in the region of 

 the hand plate into the primitive condensed mesenchyma, filling the 

 distal end of the arm bud. The same was often true of the various 

 premuscle masses. The main portion of these are quite distinct, but 

 they often shade off into each other and into the surrounding mesen- 

 chyma. We find in this embryo in the arm region that the premuscle 

 masses most closely associated with the trunk are the farthest ad- 

 vanced, those connecting the arm and trunk next, and the least devel- 



