508 DEVELOPMENT OF PRIMITIVE BODY FORM 



Hatschek, 1893; Kellicott, '13; MacBride, 1898, '00, '10; Morgan and Hazen, 

 '00; and Willey, 1894.) 



F. Early Development of the Rudiments of Vertebrate Paired 



Appendages 



Two pairs of appendages, placed at the anterior (pectoral) and posterior 

 (pelvic) extremities of the trunk, are common to all vertebrate groups. How- 

 ever, all vertebrates do not possess two pairs of paired appendages. Certain 

 lizards of the genera Pygopus and Pseudopus have only a posterior pair of 

 appendages, while in certain other vertebrates the opposite condition is found, 

 the anterior pair being present without posterior appendages. The latter con- 

 dition is found in certain teleost and ganoid fishes; the amphibian, Siren 

 lacertina; the lizard, Chirotes; and among the mammals, the Sirenia and Ce- 

 tacea. Again, some vertebrates are entirely apodal, e.g., cyclostomatous fishes 

 and most snakes, although the boa constrictors and pythons possess a pair of 

 rudimentary posterior appendages embedded in the skin and body wall. Some 

 have rudimentary appendages only in the embryo, as the legless amphibians 

 of the order Gymnophiona, and certain lizards. Consequently, the presence 

 of embryonic rudiments of the paired appendages is a variable feature when 

 the entire group of vertebrates is considered. 



The rudiments of the paired appendages also are variable, relative to the 

 time of their appearance in the vertebrate group as a whole. They are more 

 constant in the Amniota, i.e., reptiles, birds, and mammals, in time of ap- 

 pearance than in the Anamniota, i.e., fishes and amphibia. In the reptiles, 

 birds, and mammals, the limb buds arise when primitive body form is being 

 evolved. In the anuran amphibia, the anterior rudiments may appear and go 

 on to a high degree of differentiation before the appearance of the posterior 

 pair of appendages. For example, in the frog, Rana pipiens, the posterior limb 

 buds first make their appearance a brief period before the beginning of 

 metamorphosis of the tadpole into the adult form. However, the anterior 

 limb buds differentiate earlier but remain concealed beneath the operculum 

 until they become visible during the later stages of metamorphosis. In urodele 

 amphibia, the fore limb bud is not covered by an operculum, and it is visible 

 at the time of its initial appearance which occurs before the hind limb rudiment 

 arises (fig. 227J-L). 



In the majority of vertebrates, the hmb rudiment first makes its appearance 

 as an elongated, dorso-ventrally flattened fold of the epidermis, containing a 

 mass of mesodermal cells within, as shown, for example, in the chick and 

 mammalian embryos (figs. 240, 244, 246). The contained mesodermal cells 

 may be in the form of epitheHal muscle buds derived directly from the myo- 

 tomes (e.g., sharks) or as a mass of mesenchyme (chick, pig, human). (See 

 Chap. 16.) The early limb-bud fold may be greatly exaggerated in certain 

 elasmobranch fishes, as in the rays, where the anterior and posterior fin folds 



