290 COMPARATIVE ANATOMY 



they are derived from the myotomes of these segments. To the group 

 of muscles derived from this source, are added other, such as the trapezius, 

 sterno-cleido-mastoid, and levator scapulae. The pectoralis and latissi- 

 mus dorsi muscles spread out from the arm. Most of the muscles of the 

 shoulder, chest, and arm appear early in the second month, and are 

 differentiated by the beginning of the third. 



From the connexion of the muscles of the lower leg with spinal nerves, 

 including the last four lumbar and first three sacral, it may be assumed 

 that their cellular anlagen are derived from the corresponding myotomes. 

 In all essentials their development resembles that of the muscles of the 

 arm. A common mass of cells within the limb-bud differentiates into 

 dorsal and ventral muscle anlagen. The muscles from the ventral group 

 become innervated by the femoral nerve while the dorsal group are con- 

 nected with the obturator. The subdivision of the primary muscle 

 mass into the separate muscles of the adult limb is mostly completed by 

 the end of the second month. 



Visceral Muscles, Derived from the Hypomere. The visceral or 

 hypomeric include those of the heart and main blood vessels as well as 

 those associated with the alimentary canal. While most of them consist of 

 smooth muscle fibers, the visceral muscles of the head and heart are striped. 



The muscles of the wall of the alimentary canal are formed from 

 mesenchymatous cells proliferated from the visceral layer of the hypomere. 

 Such cells fill the space between the mucous epithelium lining of the 

 alimentary canal and the adjacent hypomere. They also differentiate 

 into both the connective tissues and the blood vessels of the wall of 

 the alimentary canal and into its circular and longitudinal muscles. The 

 circular layer of muscles is formed before the longitudinal layer. The 

 histogenesis of smooth muscle in some respects resembles that of striped 

 muscle. The mesenchyma cells elongate and within them rows of cyto- 

 plasmic granules are converted into myofibrils. Unlike the fibrils of 

 striped muscle, however, light and dark bands are not formed. As the 

 fibers elongate into spindle-shaped cells there is no nuclear division and 

 the fibers remain mono-nucleate. The protoplasmic bridges (plas- 

 modesms) by which the mesenchymatous cells were connected with one 

 another persist and bind the muscle fibers together. 



The fate of the hypomere in the head is much more complex than in 

 the trunk. Besides forming the heart and pericardium, the head hypo- 

 mere gives rise to the chewing muscles, the muscles of expression, and the 

 pharyngeal and laryngeal muscles. In general, the processes involved 

 are similar in lower and higher vertebrates. 



In embryos of lower vertebrates, e.g., elasmobranchs, the coelom 

 extends throughout head and trunk. In the head region, as a result of 

 the outpocketing of pharyngeal pouches, the hypomere becomes divided 



