ORIGIN AND STRUCTURE OF THE HEAD 65 



No mesomerism at all in the Vertebrate head ? — FRORlEP 

 (1882-1887) denies that there is any question of metamerism 

 in the Vertebrate head in front of the occipital region, 

 that is in front of the N. vagus. The segmentation observed 

 by Van Wyhe and others in the more anterior part of the 

 head is nothing but branchiomerism which has nothing to 

 do with mesomerism, the mesoderm in front of the vagus 

 being unsegmented and continuous from the beginning. 

 Indeed, in mammals and birds, of which representatives 

 were studied by FRORlEP, just as in Reptiles and 

 Teleosteans, the series of somites in early ontogenetic 

 stages is seen to end some distance behind the auditory 

 vesicle, all being metotic, while anterior to this point there 

 is found an unsegmented mesenchymal cell-mass, the head- 

 mesoderm of FRORlEP. Of prootic somites there is no 

 question, only in the occipital region are somites to be 

 observed. Thus it is not the fore-end of the notochord, 

 but the foramen of the vagus which forms, according to 

 FRORlEP, the boundary of two regions in the head of a 

 quite different character, the cerebral or praespinal 

 and the spinal region. The former, accordingly, comprises 

 the evertebral region of GeGENBAUR together with what 

 FRORlEP calls the pseudovertebral region, that is the region 

 of the parachordalia and of the trigeminus, facialis-acusticus 

 and glossopharyngeus and vagus. The seemingly metamerical 

 arrangement of these nerves is only a secondary consequence 

 of the branchiomerism. To the unsegmented praespinal 

 region belong the three main sense-organs of the head, 

 the olfactory organ, the eye and the auditory vesicle. 

 FRORlEP was mainly led by the study of the vagus 

 and the hypoglossus to the conclusion that reduction had 

 occurred as well at the anterior end of the occipital somites 

 as at the posterior end of the series of visceral archs, 

 and it was especially this fact which seemed to him to 

 plead for the fundamental contrast of the two regions 

 which meet here. The distinction made by FuRBRiNGER 

 (1897) of a palaeocranium and a neocranium coincides 

 with that of FRORlEP. MARCUS (1910, p. 121) and De LanGE 

 (1913), bv their researches on the early development resp. 

 of Hypogeophis and of Megalobatrachus, are led even 

 to the assumption, that the anterior head mesoderm has a 

 quite different origin to that of the segmented trunk and 

 occipital mesoderm, the former being derived from the 



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