278 Basic Structure of Vertebrates 



the midventral region, carrying with them the coelom, and meet mid- 

 ventrally to form a double vertical layer, a ventral mesentery, extend- 

 ing from the enteron to the outer body wall and separating right and 

 left coelomic cavities (Fig. 228). 



The splitting of the original sheet of mesoderm may extend dorsally 

 so far as to involve the somite, which accordingly contains a more or less 

 definite cavity, the myocoele — "myo-" because the somite is mainly 

 muscle-forming. Shortly, the somites become detached from the lower 

 somatic and visceral sheets of mesoderm and the myocoeles lose con- 

 tinuity with the permanent coelom (Figs. 229A, B; 230). Eventually, as 

 the somite differentiates, the myocoele is obliterated. 



The differentiation of the vertebrate mesoderm in the prospective 

 trunk region is more elaborate than in Amphioxus. Here, upon each 

 side, early arise three zones of differentiation: the epimeric zone, a 

 dorsal, mainly muscle-forming, region; the mesomeric zone, a kidney- 

 forming zone situated just below the former; and the hypomeric zone, 

 the most ventral, constituting the somatic and visceral layers of peri- 

 toneum (Figs. 228, 229). 



The epimeric mesoderm early undergoes a segmentation (already 

 described: p. 277) which breaks it up into a longitudinal series of paired 

 muscle-forming somites or epimeres. Each epimere undergoes three 

 kinds of differentiation. Its heavier inner wall is mainly converted into 

 striated body-muscle, not only the dorsal but also the ventral muscle. 

 The muscle-forming material grows ventralwards, pushing its way 

 between the ectoderm and the somatic mesoderm, until it reaches the 

 midventral plane (compare Figs. 230 and 231). The medial region of 

 the epimere gives rise to loosely aggregated cellular masses 

 (mesenchyme) surrounding the notochord and neural tube (Figs. 228, 

 230). This material produces such supporting structures — connective 

 tissue, cartilage, and bone — as may later be developed around these two 

 axial organs. The outer layer of the epimere breaks up to form loose 

 cellular masses, mesenchyme, which give rise to the dermis, the deeper 

 fibrous and vascular layer of the skin. 



The terms myotome, sclerotome, and dermatome are applied 

 respectively to the muscle-forming, skeleton-forming, and dermis- 

 forming regions of the epimere (Fig. 228). 



The mesomeric mesoderm gives rise to the tubular structures of 

 the kidneys. In fishes and amphibians, it undergoes a segmentation 

 dividing it into a longitudinal series of mesomeres, each adjacent to 

 an epimere (Fig. 229). Since the embryonic coelom extends into the 

 mesomeric zone, each mesomere is hollow. From each of them 

 is formed one primary renal tubule whose cavity is continuous with 

 the coelomic space within the hypomeric zone (Figs. 228, 229). In later 



