110 THE ANCESTRY OF VERTEBRATES 



exactly the same conclusion regarding the length of the 

 head of Heptanchus on embryological grounds. After having 

 treated of the development of the skull of Acanthias he 

 writes concerning that of Heptanchus: "Es §ind aber deutliche 

 Zeichen vorhanden, welche darauf hinweisen, dass die 

 Flemente zweier Wirbel mit dem Schadel verschmolzen 

 sind, so dass die beiden letzten Occipitalnerven dieses Tieres, 

 welche FuRBRlNGER mit y und z bezeichnet, eigentlich 

 Spinalnerven (Occipitospinalnerven) sind. Erst der Nerv x 

 muss mit dem oben mit z bezeichneten Nerven, der bei 

 Acanthias den primitiven Occipitalbogen durchbohrt, homo- 

 logisiert werden."^) A better confirmation of our conclusions 

 concerning the nature of the hypoglossus and its relation 

 to the gill-slits could not be given. 



Primarily and secondarily epibranchial somites and ner- 

 ves. —FuRBRlNGER, though at another place (1. c. p. 682, 

 685-686) recognizing the general homology of cranial and 

 spinal ganglia and rejecting emphatically the idea of an 

 incongruency between branchiomerism and mesomerism, yet 

 accepts Froriep's distinction of a cerebral and a spinal 

 region in the head and the idea that all the myotomes 

 belong to the latter, that is to the trunk. Thus he considers 

 the epibranchial muscles, which are to be derived from 

 the occipital myotomes, as muscles of "spinal", that is 

 primarily post-branchial, origin just as the hypobranchial 

 muscles but in opposition to the primordial bianchial muscles,- 

 the constrictor es, which he calls "cerebral" muscles and which 

 accordingly are considered as belonging to FRORlEP's 

 primarily unsegmented head region. In the same way the 

 occipital nerves must then be considered as spinal nerves. 



hi this we can not altogether agree with him. Just as 

 in Petromyzon where it has been proved by the study of 

 ontogeny, part of the epibranchial musculature of the adult 

 is no doubt of post-branchial origin and has become 

 epibranchial only by the secondary extension of the branchial 

 basket. Van Wyhe (1889, p. 558) demonstrated the 

 latter phenomenon in Selachians by determining the segment 

 in which in successive stages the biliary duct was found; 

 the latter was shown to recede into segments situated 

 progressively further backwards. In Pristiurus, embryonic 



The last occipital nerve of ylca^//j/as, however, tias already been 

 designated by FiiRBRiNOER as a, in forms like Scyllium only it is z. 



