534 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



(c) gives origin to adult notochord of Amphioxus and cyclostomatous fishes 

 and to notochordai portions of adult vertebral column of gnathostomous 

 fishes and water-living amphibia, and 



(d) also, comprises the remains of the notochord in land vertebrates, such 

 as "nucleus pulposus" in man. 



5. Mesodermal Areas 

 These areas give origin to: 



(a) Epimeric, mesomeric, and hypomeric areas of primitive mesodermal 

 tube, 



(b) epimeric portion also aids in induction of central nerve tube, 



(c) muscle tissue, involuntary and voluntary, 



(d) mesenchyme, connective tissues, including bone, cartilage, 



(e) blood and lymphoid tissue, 



(f) gonads with exception of germ cells, genital ducts, and glandular tis- 

 sues of male and female reproductive ducts, and 



(g) kidney, ureter, musculature and connective tissues of the bladder, 

 uterus, vagina, and urethra. 



6. Germ-cell Area 

 This area gives origin to: 



(a) Primordial germ cells and probably to definitive germ cells of all verte- 

 brates below mammals and 



(b) primordial germ cells of mammals and possibly to definitive germ cells. 



F. Metamerism 



1. Fundamental Metameric Character of the Trunk and 

 Tail Regions of the Vertebrate Body 



Many animals, invertebrate as well as vertebrate, are characterized by the 

 fact that their bodies are constructed of a longitudinal series of similar parts 

 or metameres. As each metamere arises during development in a similar 

 manner and from similar rudiments along the longitudinal or antero-posterior 

 axis of the embryo, each metamere is homologous with each of the other 

 metameres. This type of homology in which the homologous parts are ar- 

 ranged serially is known as serial homology. Metamerism is a characteristic 

 feature of the primitive and later bodies of arthropods, annelids, cephalo- 

 chordates, and vertebrates. 



In the vertebrate group, the mesoderm of the trunk and tail exhibits a type 

 of segmentation, particularly in the epimeric or somitic area. Each pair of 

 somites, for example, denotes a primitive body segment. The nervous system 



