DEVELOPMENT OF THE SOMITES 



15 



rate at first, but soon fuse and unite with the general coelom. Later the general coelom becomes 

 secondarily divided into the permanent pericardial, pleural and peritoneal cavities. 



The outer layer of the lateral mesodermic division forms the somatic or parietal layer of the 

 peritoneum, etc. The inner layer forms the visceral or splanchnic layer, and develops not only 

 the serous membrane, but also the muscular and connective tissue of the walls of the alimentary 

 canal and its derivatives. 



Development of the somites. Metamerism. — ^The appearance of the somites 

 marks the beginning of metamerism, the arrangement of the body in successive 



Fig. 18. — Diagram of a Cross Section of a Human Embryo. 



Spinal cord 



' •e*> ■v>x Mesodermic somite 



Notochord 



Ventral mesoderm, visceral layer 



Intermediate cell mass 



Ventral mesoderm, 

 somatic layer 



segments or metameres. Each somite develops a primitive muscle segment, 

 myotom,e, and a skeletal segment, sclerotome (figs. 18, 19). Moreover, the cor- 

 responding nerves and blood-vessels likewise assume a metameric arrangement. 

 This metamerism persists (more or less modified) in the adult neck and trunk. 



The differentiation of the somites is illustrated by figs. 18 and 19. The medial wall of each 

 somite forms the sclerotome. Its cells migrate to form the corresponding vertebra, rib, etc., as 



Fig. 19. — Diagrams Illustrating the History op the Mesoderm. M, myotome, dM, 

 dorsal portion of myotome. vM, ventral portion of myotome. SC, sclerotome, gr, genital 

 ridge. Wd, Wolffian duct. Sm, somatic layer of mesoderm, vm, visceral layer of mesoderm. 

 mr, membrana reuniens. /, intestine. N, neural tube. (McMurrich.) 



well as the mesenchyme forming the various connective tissues in this region. The remainder of 

 the somite forms the myotome, from which the voluntary musculature of the trunk, the neck and 

 (in part) the head is derived. The dorsal portions of the myotomes develop the muscle in the 

 dorsal region of the trunk, while the ventral portions extend ventralward to form the muscula- 

 ture of the latero-ventral body walls (figs. 19, 20, 21, 23). 



At the junction of the dorsal and ventral divisions of the mesoblast is a group of cells called 

 the intermediate cell mass. This mass becomes segmented (corresponding to the somites) and 



