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of the cranium was segmented in a previous period of the phylo- 

 genelical development ? 



In this last foi'm the problem is at the present moment still benig 

 discussed, though the arguments that are now brought forward to 

 enable us to come to an affirmative answer of this question, are of a 

 character quite different from those which Gegknbaur, who was the first 

 to formulate it in this way, developed for it. At present the state of 

 the problem is indeed so, that a positive answer of the (|uestion is 

 no longer contested by any of the investigators, and they only do 

 not agree in stating how great tiie part of the cranium is, over 

 which the mentioned segmentation extends. 



In connection with the much earlier ontogenetical investigations 

 of Rathke, Gegenbaur distinguished in the cranium 2 parts, a frontal 

 not segmented part and a posterior segmented part. The two parts 

 are designated as the vertebral part and the praexertebral one. 



According to Gegenbaur, who formed his theory from the pheno- 

 mena of the Selachier-ci'anium, the vertebral part would form by 

 far the greater part of the cranium ; oidy the region in which the 

 N. opticus and the N. olfactorius pierce through the skull, would 

 belong to the praevertebral region. The vertebral part constructed 

 by fusion of about 9 cranial vertebrae would be primary, and it 

 is only after concrescence of these elements, that the praevertebral 

 jiart would have been developed by excrescence in a frontal direction 

 of liie cai-tiiageous part formed in the above mentioned manner, 

 under adaptation to the olfactory groove and the optical organ. 



We do not find with Gegenbaur a primitive part of the cranium, — 

 principally to be distinguished from the other segmented part of the 

 cranium — , which ought to be maintained as real primordial cranium 

 contrary to the vertebral column. The body of vertebrates consisted 

 of a number of equivalent segments. The frontal part of these has 

 fused for the formation of the cranium, the posterior part forms the 

 vertebral column. Secondarily, by excrescence, an unsegmented part 

 has still been added to the segmented part of the cranium. 



Stöhr added to this the opinion that the number of segments 

 used for the construction of the cranium is not constant, and con- 

 tinually increases in the series of vertebrates. The craniovertebral 

 interval shifts consequently more and more in a caudal direction. 

 Other investigators could confirm the correctness of this view. 

 Sagemehl succeeded in showing, that the cranium of higher developed 

 pisces and of amnioles has incieased in a caudal direction with 3 

 vertel)rae. This cranium would consequently be the Selachiercranium 



