Warren Harmon Lewis ISl 



digit. The extensor indicis muscle joins the nlnar side of the tendon 

 of the commimis to the second digit. 



The Nehves.— The drachial plexus has a decided posterior inclina- 

 tion and seems to have been pulled down against the first rib. The three 

 cords are so close together that it was impossible to separate them satis- 

 factorily though indications of the cords are present. There is nothing 

 especially peculiar about the distribution of the nerves from the plexus^ 

 either motor or sensory, which is not present in the adult. 



Summary. 



The first indications of the arm bud appear during the third week as 

 a slight swelling in the lower cervical region on the anterior portion of 

 the Wolffian ridge. This gradually enlarges and by the time the embryo 

 is 4.5 mm. in length and three weeks old the arm is of considerable size. 

 The base now lies opposite the lower four cervical and first thoracic ver- 

 tebrae. The arm bud is at first filled with a homogeneous and closely 

 packed mesenchyma. No nerves or myotome buds have entered the 

 arm, yet it contains the tissue from which the muscular and skeletal 

 elements develop. 



During the fourth week before the nerves enter differentiation begins 

 by an increased condensation for the skeletal core. The nerves, how- 

 ever, have reached the base of the arm and have united by their ex- 

 panded ends into the first beginnings of the cervico-brachial plexus. 

 During the fifth week the nerves from this plexus push into the pre-, 

 muscle sheath which surrounds the skeletal core. 



By the end of the fifth week the skeletal core can be differtotiated 

 into many of the skeletal elements, three of Avhich contain cartilage, 

 namely, the humerus, ulna and radius. The premuscle sheath also has 

 become more or less differentiated into muscles or groups of muscles, 

 between which, however, no sharp lines can, as a rule, be drawn. 

 Toward the distal end the differentiation is less complete, and in the 

 hand premuscle tissue still represents the intrinsic muscles. The nerves 

 have grown into the hand and spread out in a veiy peculiar manner. 

 Most of the branches of the brachial plexus found in the adult are now 

 present. 



By the end of the sixth week most of the muscles of the arm are 

 easily recognized. The intrinsic muscles of the hand are just beginning 

 to show fibrillation and are still mostly of premuscle tissue. The ten- 

 dons and ligaments are also becoming more sharply differentiated. 

 Most of the skeletal elements consist of cartilage and the surroundino- 



