64 THE ANCESTRY OF VERTEBRATES 



agreement as to the number of segments, since several 

 investigators admit the possibility, that gill-clefts may have 

 fallen out 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. ZlEGLER^ 

 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- 

 ponds 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. ZlEGLER, 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 former 

 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. 



