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 



eet. 



Fig. 23. Transverse section of a rabbit-embryo of about loi days, 

 sliowing tlie arrangement of tiie organ-systenis : ao., aorta; ch., not- 

 ochord; coe., coelomic cavity; d.m., dorsal mesoderm (myotomic 

 and sclerotomic divisions); e., primitive alimentary canal (enteron); 

 ect., ectoderm; l.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 



