ORIGIN AND STRUCTURE OF THE HEAD 117 



scLired by FiiRBRlNGER's voluminous work of the latter year: 

 "Nimmt man an, dass es eine Stammform gegeben hat, die 

 in Bezug auf den Occipitalteil des Schadels sich wie die 

 Amphibien verhielt, so wiirde sich weiterhin die Ent- 

 wickelung in zwei getrennten Bahnen bei Fischen und Land- 

 wirbeltieren bewegt haben". Few, i think, will follow Rabl 

 (1888, p. 656) in supposing that first in the Elasmobranch 

 egg an accumulation of yolk has occurred which, however, 

 in the Amphibian egg has been lost again for some 

 unknown reason, has been reacquired in Sauropsids and lost 

 for the second time in viviparous Mammals. Especially if 

 we compare the early stages of development of Petromyzon, 

 Elasmobranchs and Amphibia, it will be evident at once 

 that in this respect also the Elasmobranchs represent a side- 

 branch, however primitive they may be in other characters. 

 In the structure of the brain also the Amphibia stand the 

 nearer to Petromyzon, a metencephalon having developed 

 in neither of them, unlike Selachians and Amniotes. In the 

 former two the hypophysis originates from the ectoderm in 

 front of the mouth, in the latter two from the roof of the 

 mouth-involution. Concerning the development of the cranial 

 muscles also, EDGEWORTH (1911) finds that the Amphibians 

 exhibit more primitive features than the Selachians. 



We could no doubt keep to the names proto- and auxi- 

 metameric neocranium to express the difference in length 

 of the Amphibian skull on the one hand and the skull of Am- 

 niotes on the other. However, we should then be using these 

 terms in a quite different sense from that attributed to them 

 by FURRRINGER, for in the protometameric neocranium have 

 now been incorporated only the glossopharyngeus and the 

 vagus and in the auximetameric neocranium only the nerves 

 y, z, while the nerves a, b, c, the occipito-spinal nerves, at 

 least in Amniotes, do not exist as such. Only if with FuR- 

 BRINGER (1897, p. 362) we designate the last occipital 

 nerve in Acanthias as a, have we probably to do so in 

 Amniotes also, as will be shown at the end of this chapter. 

 According to FiiRBRINGER a protometameric neocranium is 

 found e. g. in Scyllium. If, however, we use his terms in 

 the sense mentioned above, Scyllium has an auximetameric 

 cranium. A somewhat different terminology will be propo- 

 I sed at the end of this chapter. 



Before combining the results of the above consider- 

 ations into a general survey of the history of the head of 



