THE MUSCULAR SYSTEM 



319 



Lewis, "there are no observations of distinct myotome buds extending into the 

 limbs." A diffuse migration of cells from the ventral portion of the myotomes 

 has been recorded by various observers, recently by Ingalls. These cells soon 

 lose their epithelial character and blend with the undifferentiated mesenchyma of 

 the limb buds (Fig. 323). From this diffuse tissue, which at about 10 mm. forms 

 premuscle masses, the limb muscles are differentiated, the proximal muscles being 

 the first to appear (Fig. 322) . 



Spinal ganglion 



Dermatome 



Myotome 

 Spinal nerve 



Arm hud—S&, 



Proliferating cells of. 

 myotome 



Mesonephric duct 



Mesonephric tubule and 

 glOmeruhis 



Cmlom 

 Somatic mesoderm- 



Fig. 323. — Transverse section of a 10.3 mm. monkey embryo showing the myotome and the mesenchyma 

 of the arm bud (KoUmann). 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 myotomic segments having 

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



The remaining muscles of the head differ from all other skeletal muscles in 

 that they arise from the splanchnic mesoderm of the branchial arches and are 

 innervated by nerves of a different category than those which supply myotomic 

 muscles. The mesoderm of the first branchial arch gives rise to the muscles of 

 mastication and to all other muscles innervated by the trigeminal nerve. Simi- 



