178 F. H. EDGE WORTH. 



If, following' Furbi-iuger, it be supposed tiiat primitively 

 the myotomes lay exclusively lateral to the claorda dorsalis, it 

 would follow that they have taken different paths of develop- 

 ment in the head and body, resulting in the conditions above 

 stated. 



According to the generally accepted theory, certain 

 Selacliii, e.g. Heptauchus, are the most primitive of gnatho- 

 stome Vertebrates, in that they have the greatest number of 

 branchial seg^ments ; and the lessened number of branchial 

 segments in Teleostomi, Amphibia, and Dipnoi is due to a 

 disappearance of the hinder ones. If, however, it be supposed 

 that the original number of branchial segments was two, i.e. 

 first and second, and that these were added to by a process of 

 metameric increase, the interesting question arises as to the 

 least number present in these Vertebrate groups, for this may 

 be supposed to have been possessed by some primitive form. 

 The Amphibia have four branchial segments. Dipnoi and most 

 Teleostomi five (though Polypterus has only four), and most 

 Elasmobranchs five, though they may have as many as seven. 

 It may therefore be supposed that the original number 

 present in Amphibia was added to in the other groups. This 

 harmonises with the conclusion, stated later, that the condition 

 of the muscles of the head in Amphibia is more primitive than 

 in Dipnoi, Teleostomi, and Elasmobranchs. 



The lessened number present in Sauropsida and Mammalia 

 may be supposed to have resulted from reduction of the hinder 

 of these four branchial segments. In the later stages of 

 Reptiles, previously investigated, only two branchial myotomes 

 were seen, but the early stages of Chrysemys niarginata 

 show four. In Gallus only two are present, even in the early 

 stages. In Lepiis only the first three are developed. 



The cephalic coeloni disappears in the mandibular and 

 hyoid segments early in development, and its walls develop 

 into the intermandibularis and interhyoideus, which are at 

 first continuous with the mandibular and hyoid myotomes. 

 The lower ends of the branchial myotomes separate from the 

 wall of the branchial portion of the cephalic coelom, and they 



