THK MUSCULAR SYSTEM 



319 



nous observers, recently by Ingalls. These cells soon lose their epithelial 

 character and blend with the undifferentiated mesenchyma of the limb 

 buds (Figs. 322 and 323). From this diffuse tissue, which at about 9 mm. 

 forms premuscle masses, the limb muscles are differentiated, the proxi- 

 mal muscles being the first to appear. The progressive differentiation into 

 distinct muscles reaches the level of the hand and foot in embryos of 

 20 mm. 



Spinal ganglion 



Dermalome 



Myotome 

 Spinal nerve 



Arm bud 



Proliferating cells of 

 myotome ' 



MesQnephric duct 



Mesoncphric tubule and 

 glomerulus 



Coelom 

 Somatic mesoderm 



^ V t>~ v 



FIG. 323. Transverse section of a 10.3 monkey embryo, showing the myotome and the mesen- 

 chyma of the arm bud (Kollmann) A, aorta; *, sclerotome. 



Muscles of the Head. Distinct mesodermal segments do not occur 

 in the head region. It is possible, however, that a premuscle mass, from 

 which the eye muscles of man are developed, is comparable to three myo- 

 tomic segments having a similar fate in the shark (cf. p. 366). 



The remaining muscles of the head differ from all other skeletal mus- 

 cles in that they arise from the splanchinc mesoderm of the branchial 

 arches and are innervated by nerves (visceral) of a different category than 

 those (somatic) which supply myotomic muscles (p. 356). The meso- 

 derm of the first branchial arch gives rise to the muscles of mastication and 

 to all other muscles innervated by the trigeminal nerve. Similarly, the 



