Charles Eussell Bardeen and Warreu Harmon Lewis 35 



The structures of the upper arm and thigh are differentiated before 

 those of the forearm and leg, and the latter before the hand and foot. 

 Differentiation in the anterior limb precedes that in the posterior limb. 

 Most of the main structures of the anterior limb may be distinguished 

 at the end of the sixth week; most of those of the posterior limb at the 

 end of the seventh week. 



During the first two months of embryonic life, therefore, are developed 

 the rudiments of the muscles, nerves, blood-vessels, and skeletal struc- 

 tures characteristic of the back, the body-wall and the limbs. Adult 

 conditions are reached by an increase in size and complexity of the 

 various organs and by a relative shifting of parts. 



