116 THE ANCESTRY OF VERTEBRATES 



Ontogeny teaches us that the occipital arch behaves 

 at first exactly like a vertebral arch, originating in ^he 

 myocomma between two of the post-otic myotomes and 

 differing only from the neural archs following behind it in 

 that it afterwards fuses with the auditory capsule, leaving 

 the foramen vagi between the latter and itself. Much better 

 founded, therefore, seems to me to be the view of SToHR 

 (1879, 1881) and Sewertzoff (1895, p. 252) that the 

 occipital arch in Amphibians actually represents the neural 

 arch of only one vertebra which according to SEWERTZOFF 

 corresponds to the first free neural arch of Petromyzon. 

 Truly, we must then assume with SEWERTZOFF (1897, p. 

 410): "Bei Amphibien, wie ich hierbei hervorheben mochte, 

 entspricht der ganze Occipitalabschnitt einem einzigen 

 Segmenten, dem einfachen Occipitalbogen, so dass die 

 Amphibien in dieser Hinsicht unter alien Cranioten, mit 

 Ausnahme der Petromyzonten, die einfachsten Zustande 

 zeigen." While the skull of Petromyzon, as stated above, 

 comprises, besides the prostomium, only two segments, 

 the trigeminus- and the facialis-segment, this number evi- 

 dently amounts to five in Amphibians, the meta-otic glos- 

 sopharyngeus-segment and those of the primary vagus 

 and of the "spinalartiger Vagusanhang" having been 

 added to the cranium. This conception of the Amphibian 

 cranium is strongly supported by the recent researches of 

 Miss Platt (1897) on Necturus and of GOODRICH (1911) 

 on Siredon. In Elasmobranchs and Amniotes some two or 

 three, sometimes even a few more, segments have been 

 incorporated into the occipital region. We shall revert 

 to this at the end of the chapter. 



The halo of primitiveness which since Gegenbaur, 

 Balfour and Van Wyhe surrounds the head of the Elas- 

 mobranch thus looses part of its glory. If, however, we 

 call to mind that in no other group of Vertebrates do the 

 structure of the egg and the early stages of development 

 show so close an agreement with those of Cyclostomes 

 as is the case in Amphibians, that especially in Urodelans 

 the similarity is truly striking and that on the other hand 

 the Elasmobranchs in these respects show a deviation from 

 this simple type which nearly equals that found in Saurops- 

 ids, we shall have less difficulty in accepting the view 

 expressed by SEWERTZOFF (1897, p. 410) in two articles 

 (1895, 1897) of which the influence has been greatly ob- 



