Motor System — Muscles and Skeleton 91 



of the adult. In later embryonic stages of other vertebrates, there is 

 more or less modification of the segmental arrangement of the muscle- 

 forming material. In amphibians and reptiles the horizontal septum is 

 at a somewhat higher level than in fishes (Fig. 91A-B). In adult birds 

 and mammals there is no definite septum (Fig. 91C). The epaxial mus- 

 cles become concentrated along the vertebral column, with whose 

 movements they are chiefly concerned, leaving the hypaxial regions of 



Fig. 90. Diagrammatic transverse section 

 of the body of a vertebrate embryo at an ad- 

 vanced stage. The muscle-forming myotome 

 is beginning to extend into the ventral body- 

 wall of the embryo, (c) Coelom; (g) genital 

 ridge; (m) muscle derived from myotome; 

 (mc) myocoele; (p) peritoneum; (pd) pro- 

 nephric duct; (so) somatic layer (derma- 

 tome) of somite; (») advancing ventral bor- 

 der of myotome. The finely dotted areas are 

 occupied by mesenchyme. (Courtesy, Kings- 

 ley: "Comparative Anatomy of Verte- 

 brates," Philadelphia, The Blakiston Com- 

 pany.) 



the myomeres to become the relatively thin layer of muscle of the 

 lateral and ventral walls of the coelom. Fusion of parts of successive 

 somites produces long muscles extending over distances of several or 

 many of the original segments. Broad, thin sheets of muscle, such as 

 occur in the abdominal walls (Fig. 91), may be formed, usually showing 

 in the adult no trace of segmentation. In all vertebrates, however, the 

 primary segmentation is retained in systems of short muscles such as 

 those extending from one vertebra to the next or between two adjacent 

 ribs. 



The point of special significance is that, in all vertebrates from fish 

 to man, the embryonic mesoderm which produces body-muscle (with 



