250 
agreement as to the number of segments, since several 
investigators admit the possibility, that gill-clefts may have 
tallen our or have changed their function and character. Thus 
MARSHALL (1879) supports the view, suggested already by 
DOHRN (1875) in his theory, that the olfactory grooves are- 
modified gill-clefts and the olfactory nerve a segmental nerve, 
while according to VAN WYHE (1882) the hyoid arch corres- 
ponds to two segments, since he finds two somites over it, 
a gill-slit apparently having fallen out here. VAN WYHE rejects: 
MARSHALL's conclusion and MARSHALL that of VAN WYHE, 
while BEARD (1885), in a somewhat modified form, accepts 
both. He sees in the N. facialis and the acusticus the two- 
dorsal nerves belonging to the double hyoid segment and 
considers both the olfactory pits and the auditory vesicles. 
not as modified gill-slits, but as their branchial sense-organs, 
the gill-slits themselves having atrophied. DOHRN even 
assumes a very considerable number of gill-clefts to have 
fallen out, or to have been transformed into olfactory groo- 
ves, hypophysis, thyroid gland, auditory vesicles, etc. ZIEGLER, 
on the contrary, doesnot admit any falling out of gill-slits 
at all, thus arriving at a very simple scheme, which we 
will refer to later. That the mouth, in its relation to the 
somites and the cranial nerves and their ganglia, corres- 
onds to a pair of gill-slits, is a point upon which 
all agree, though the view, first advocated by DOHRN, 
that the mouth has originated from the union of two 
gill-clefts, is less generally accepted (cf. e.g. ZIEGLER, 1908). 
Branchiomerism independent from mesomerism? — Turning 
now to the other school of thought, we find that AHLBORN 
(1884 p 321, 322) was one of the first to deny the correspon- 
dence between branchiomerism and mesomerism, the former 
taking its origin from the entoderm, the latter from the 
mesoderm. According to him GEGENBAUR' s comparison of the 
visceral archs to the ribs does not hold, the latter originating 
intersegmentally, in the intermuscular ligaments, the forimer 
within the segments delineated by the gill-pouches. The meta- 
meric arrangement of the ribs is the expression of the primary 
mesomerism, that of the gill-bars of the entodermal branchio- 
merism. Yet, though the latter is independent of the former, 
AHLBORN does not deny that mesomerism reaches into the head, 
in this respect he wholly supports VAN WYHE's views. The 
mesomerism, however, observed dorsally, and the branchio- 
merism, observed ventrally, are independent of each other. 
