46 
different groups; just as much can this be the case with 
the segments to which the branchial and the lu:mbal plexus 
belong. In the year 1879 FüRBRINGER (p. 389), as a result 
of his researches on the plexus brachialis and lumbosacralis, 
came to the conclusion that these plexus are not bound to 
definite myotomes, but that their situation and extent depend 
on the situation and development of the limbs they innervate. 
If the limb be strongly developed the number of segments 
participating in supplying this with muscles and nerves may 
increase and, in the opposite case, it may decrease, though 
no intercalation of new segments, or falling out of segments 
formerly present, can be assumed to account for this. In 
the same way the shifting forwards or backwards of the 
limbs is not the result of a moving of corresponding 
segments but adjoining segments take over the task of the 
old ones which now come free. GOODRICH (1914) has 
recently contributed a very interesting article on this subject 
and has shown plainly that there is no primary relation 
between homology and metamerism. We can speak only 
of a regional homology, the homology of a region or 
segmental level which may move caudad and rostrad 
and may extend over a greater or lesser number of 
segments. 
The anterior limit of the hypoglossus-level is determined 
by the situation of the last gill-slit, i. e. by the number 
of gill-slits or the extent of the branchial level. From this 
and from the backward extent of the skull depends whether 
the hypoglossus will lie far behind the cranio-vertebral limit 
(Petromyzon,) whether its anterior limit will nearly coincide 
with it (Amphibians and several Elasmobranchs), or whether 
the hypoglossus will be for a lesser or greater part intra- 
cranial (other Selachians, Amniotes). It also depends on the 
situation of the last gill-slit whether the hypoglossus-region 
will be found far behind the vagus and beyond its sphere 
of influence which causes the dorsal roots and spinal ganglia 
behind it to atrophy (Petromyzon) or whether it will reach 
with its anterior end into the sphere of influence of the 
vagus (Selachians, Amniotes). 
If then at least part of the occipital myotomes and their ner- 
ves are to be considered as belonging primarily to the branchial 
region and, as argued before, FRORIEP’s distinction between 4 
cerebral unsegmented and a spinal region of the cranium must 
be rejected, then our conception of the occipital region becomes 
