89 
without the somites themselves moving in one direction 
or in the other or fusing or being split up. We have to 
do simply with a displacement of differentiation and not 
of the cell material itself. 
The hypoglossus-musculature and the hypoglossus itself, 
originating always from the anterior post-branchial somites 
and having a constant relation to the branchial basket 
which has neither always the same extension, must 
therefore be considered as homologous throughout the 
whole series of Craniates, from the Cyclostomes on to the 
Amniotes. The length of the skull, however, being inde- 
pendent from the number of gill-slits, i.e. from the length 
of the branchial segmental level, the cranio-vertebral limit 
has no constant relation with the hypoglossus-roots. In 
the same way e.g. the paired and the unpaired fins in 
different groups of fishes may vary in their situation inde- 
pendently from each other. 
Dr. H. C. DELSMAN. 
(to be continued). 
