Warren Harmon Lewis 153 



some advances over LXXX. The arm bud is filled with the closely 

 packed mesenchyma similar to that seen in LXXVI and LXXX, with 

 this important difference however that in the center of the mass the 

 cells are somewhat more closely packed than at the periphery, and repre- 

 sent the first beginnings of differentiation in the arm. This probably 

 represents the hnmerns. It will be seen from Fig. 5 that the peculiar 

 tissue in the arm bud has spread some distance into the membrana 

 reuniens. Here, as in the previous stages it is impossible to determine 

 whether cells may not go from the myotomes into the arm bud. The 

 nerves, as in LXXX, pass along the median side of the myotome without 

 sending branches into the myotome or, so far as I can determine, taking 

 a portion of the myotome along into the arm bud. They extend farther 

 into the arm than in the preceding stage. The beginning of the cer- 

 vical and brachial plexus is even more marked, and is formed by anas- 

 tomoses of the brush-like ends of the first cervical to the second thoracic. 

 The root of the arm lies at the level of sixth cervical to the second 

 thoracic intervertebral discs. 



SUMMAET. 



The tissue from which the muscles, ligaments, tendons, and carti- 

 lages of the arm develop is present at a very early stage in the arm bud, 

 probably by the beginning of the third week. No distinct myotome 

 buds take part in the formation of this tissue. That cells from either 

 or both myotomes and somatopleure enter into this early arm mesen- 

 chyma cannot be determined from the material at my disposal. If cells 

 do migrate from the myotomes, they apparently do so independently of 

 the nerves which are not present until the tissue is formed and fills the 

 arm bud. The first beginnings of differentiation of this peculiar tissue 

 which fills the arm bud occur durine; the fourth week. 



PART 11. 



The Diffeeentiation of the Mesenchyma of the Aem Bud into 

 muscrlae and skeletal elements, and the 



GeOVYTH of THE NeEVES. 



Embryo CLXIII. 



The first indication of a differentiation of the mesenchyma of the 

 arm bud occurs as we have seen in an embryo of about four weeks. In 



