251 
No mesomerism at all in the Vertebrate head ? — FRORIEP 
(1882-1887) denies that there is any question of metamzerism 
in the Vertebrate head in front of the occipital region, 
lk er in front of the N. vagus. The segmentation observed 
AN 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 FRORIEP, 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 FRORIEP. 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 foums, according to 
FRORIEP, 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 
FRORIEP calís 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 prespinal 
region belong the three main sense-organs of the head, 
the olfactory organ, the eye and the auditory vesicle. 
FRORIEP 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 cf 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 FüRBRINGER 
(1897) of a palaeocranium and a neocranium coincides 
with that of FRORIEP. MARCUS (1910, p. 121) and DE LANGE 
(1913), by 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 
