ORIGIN AND S TRUCTURE OF THE HEAD 63 



the latter probably is to be considered as a secondary 

 outgrowth from the former. The vertebral part only is 

 traversed by the notochord, and the number of vertebrae 

 the former contains is indicated by ihe number of visceral 

 archs — seven in the case of pentanch Elasmobranchs — , 

 to which originaliy tv/o arches in front were added by 

 Gegenbaur. represented by the labial cartilages. 



Ontogenetic researches. — As might be expected, since 

 Gegenbaur a great number of investigators have tried to 

 rediscover in ontogeny ihe metameric structure so much 

 obscured in the adult Vertebrate head. The results of their 

 researches, however, have not yet led to unanimity « f opi- 

 nion, neither in details nor even in fundamental questions. In 

 regard to the latter we may distinguish two schools of thought 

 viz : those who in the main adhere to GeGENBAUR's opinion,^ 

 that the arrangement of the gill-clefts corresronds to the S 

 number of head segments and that the segmental structure 

 reaches as far as the fore-end of the notocnord, and those ' 

 who reject this view and consider only the occipital region 

 of the neurocranium to be derivable from segments like 

 those of the trunk. According to the former the situation ^ 

 of the gill-slits is intersegmental, each having broken 

 through between two mesoderm segments; according to 

 the latter the mesoderm in front of the vagus shows no 

 metameric segmentation at all, but is unsegmented, and 

 only the piercing of the gill-slits makes it appear segmen- 

 ted, causing a kind of pseudo-segmentation, the branchio- 

 merism, which has nothing to do with the mesomerism 

 of the trunk. To the adherents of the first view belong 

 those investigators especially who studied the development 

 of Elasmobranchs, like BALFOUR (1878, p. 211), MiLNES 

 MARSHALL (1879. 1882). VAX WYHE (1882, 1889), BE.ARD 

 (1885), DOHRN (1881-1902 », SEWERTZOFF (1899), ZlEGLER 

 (1908) and his disciples, while those ot the second view, 

 put forward esoecially by Froriep (1882, 1887), were led to 

 their opinion mainly by the study of other Vertebrates, such 

 as Amphibians and Amniotes. 



Mesomerism, neuremerism and branchiomerism. — The first^ 

 group of investigators agrees with GEGENBAUR in that, in gene- 

 ral, the number and arrangement of the gill-slits corresponds 

 to that of the somites, and that there is a coincidence of branch- 

 iomerism, myomerism and neuromerism. Truly, there is no 



