GENERAL ORGANIZATION. 47 
systems are grouped in a characteristic fashion about it (Figs. 21, 23). 
(2) Metamerism. A large portion of the body, mainly 
dorsolateral in position, is arranged on a segmented plan, in which 
parts are repeated serially and longitudinally around and to either 
side of the original axis. Metamerism does not appear to any 
extent on the surface of the adult body, but becomes evident in 

Fic. 23. Transverse section of a rabbit-embryo of about 10} days, 
showing the arrangement of the organ-systems: ao., aorta; ch., not- 
ochord; coe., coelomic cavity; d.m., dorsal mesoderm (myotomic 
and sclerotomic divisions); e., primitive alimentary canal (enteron); 
ect., ectoderm; |.b., limb-bud; ms., mesenchyme; my., external portion 
of a myotome; n, nephrotome of embryonic kidney; intermediate mass 
of mesoderm; sp. and so., splanchnic (visceral) and somatic (parietal) 
portions of the ventral mesoderm. 
structure in the subdivision of the vertebral column into vertebrae 
and the paired, serial arrangement of the related spinal nerves, 
vessels and musculature. Metamerism is externally evident in 
embryos (Fig. 22) and is anatomically founded upon the serial 
arrangement of parts of the mesoderm. 
(3) Branchiomerism. This is an adult feature of lower 
aquatic vertebrates (Fig. 30), such as fishes, where it appears as a 
