150 



The Development of the Arm in Man 



first thoracic intervertebral disks. The cells of the median lamella of 

 the myotomes have been converted into muscle fibers. The myotomes 

 are fairly well defined and do not show buds, or, so far as I can deter- 

 mine, migration of their cells into the arm bud. The general trend 

 of the growing ventral end of the myotome is not out towards the arm 

 bud, biit ventrally towards the ccelom. It will be seen in Fig. 3 that a 

 considerable portion of the root of the arm lies close to the dorsal end 

 of the ccelom and that a proliferation of cells from its lining might 

 easily contribute to the arm tissue. The spinal nerves are not formed 

 though a few anterior root fibers appear to pass directly lateral from 

 the anterior horn. Most of them are lost in the surrounding mesen- 



Sth cervical =«j'S^(S%^,ob jS^ 

 muscle plate .•^WiJfel^tS .felST»=- 





somaiopleure 



Wolffian duct. 



V#*%. 



muxcle plate AMii'tif-^-^;,-^ 



■^'■ISk '■' ■"■■'\ ' '^' 



ai-m bud j/}^J}'^^M,>'X'%-('-% '""''^ A 



■'-^~r'.'X%J-i-;y j' 



border vein 



Fig. 3. 



•I 



Fig. 3. 



Fig. 2. Cross section through the eighth cervical myotome of embryo 

 ,XII. X 100 diameters. 



Fig. 3. Cross section through the eighth cervical myotome of embryo 

 LXXVI. X 50 diameters. 



chyma, a few, however, appear to reach the group of muscle fibers on 

 the median surface of the myotomes. This is an exceedingly important 

 stage. The arm bud is filled with a peculiar closely packed mesenchyma 

 which, so far as I am able to judge, is the same sort of tissue from which 

 at a later stage the skeletal and muscular tissues differentiate. This 

 tissue fills the arm before the nerves are developed, and if there are 

 cells from the myotomes present they have migrated there without the 

 nerve supply. 



In Embryo LXXX, 5 mm. in length, the arm bud has increased con- 



