152 



The Development of the Arm in Man 



as along its course by loose mesenchyma which is quite different from 

 that in the arm bud, but like that which lies between the myotome and 

 the aorta, and through which the nerve has pushed, probably carrying 

 some of this tissue with it and before it. The first beginnings of the 

 cervical and brachial plexuses are present in the form of anastomoses 

 of the brush-like ends of the first cervical to the third thoracic. It is 

 an interesting fact that at this stage the upper thoracic myotomes extend 

 to a considerable distance ventral of the ventral union of the arm bud 



sp. gang_ y/yjjTrk:^^ 



'neural process 



condensed m^s 



Sth muscle Plateilpf^.^^V/'h/ • 



Sth nerve #i^FH^>^^■^■ ' ' ' ''^ " / 

 Ttli nerve ^,,jt^.i,Sl-im^li\l:^>i:^i^ 





border ^eln 



intervertebral disc 



^ -jom iiopleure 



Fig. 5. Cross section through the eighth cervical myotome of embrj^o 

 II. X 50 diameters. 



with the body wall. The tip of one of these thoracic myotomes is seen 

 in Fig. 4. In the cervical region, on the contrary, the myotomes are 

 much shorter and do not extend so far ventrally. 



In the Buxton embryo about the same conditions exist as in LXXX. 

 Embryo Buxton is 5 mm. in length and about 25 days old. It was 

 stained in haematoxylin and eosin, thus bringing out the muscle plates 

 even better than with the alum carmine in which Dr. Mall's embryos 

 were stained. 



Eiribryo II is 7 mm. in length, and about four weeks old. It shows 



