m 



augmented witli 3 vertebrae. With I'sgard to the formation of thé 

 Selaohiercranium Sa(;kmkhi. is of the same opinion as Gegenbauek. It 

 wonld namely have taken existence from metameres. It is howevei 

 of great importance to remark iiere, that, according to Sagemfjil, 

 tliese metameres liad not yet the character of vertebrae, and that 

 conseqnentiy the fusion-progress of these metameres in order to form 

 the Seiachier-cranium is not equivalent to tlie addition of the H 

 vertebrae to the Seiachier-cranium, which we observe with higher 

 pisces and amniotes. 



Sagemehl calls the Selachier cranium protometamere, tlje cranium 

 enlarged by the addition of 3 vertebrae auximetamere. 



Van Wijhe showed that with Selachiers 9 segments (primordial 

 vertebrae, somites) can be distinguished at the dorsal head mesoderm, 

 wh'ch cori'espond entirely with and are equivalent to (hose of the 

 trunkregion. Gegenbauer's vievs', that the head would be nothing 

 else but a transformed part of the trunk, was certainly supported 

 by this discovery. Va.n' Wijhe's discoveries were however not of 

 such great signiiicalion for the skeleton, as he could show, it is 

 true, that sclerotomes originated from the primordial vertebrae, but 

 it appeared likewise from his investigations, tiiat this segmentation 

 of the primitive formation of the skeleton was immediately again 

 suppressed. 



The investigations of Frouiep are of great importance for the 

 problem of the cranium metamery. 



Froriep likewise distinguishes 2 [larts of the cranium, one formerly 

 segmented part and one unsegmented part. In this respect he con- 

 sequently agrees with Gegenbaur. Not so however with regard to 

 the place of the boundary-line between the two regions. According 

 to Gegenbaur this boundaiy-line would be situated far frontally, and 

 the unsegmented |)art would be restricted to the part of the cranium, 

 formed secondaiily in the neighbourhood of the olfactory groove 

 and the optical oi'gan. Froriep however admits as boundary-line 

 between the two regions the S[)0t, where the N. Vagus pierces through 

 the base of the skull. The earlier segmented part is thus, according 

 to Froriep, but very small and confines itself only to the occipital 

 region. Froriep showed now that with cow and hen this occipital 

 part behaves ontogenetically as the frontal part of the vertebral 

 column, and consequently shows likewise the design of primordial 

 vertrebrae, vertebral arches and nerves, whilst in the region lying 

 before the vagus nothing is |)erceptible that could be compared to 

 the segmentation in the spinal trunk-region. In accordance iierewith 

 Froriep distinguishes in the ci-anium a s|)inal and a |)iaespinal part. 



