52 
Vertebrates we must first consider yet another question, viz. 
that ofthe relation of the head of Craniates to that of Amphioxus. 
Head of Amphioxus — Attempts to derive the general 
structure of de Craniate head from what is found in Amphioxus 
have not been wanting. GEGENBAUR (1872, p. 300, 1887, 
p. 98) was the first to point to Amphioxus as the form in 
which we see the metameric structure of the trunk directly 
and uniformly continued into the head. He consicers the 
whole branchial region of Amphioxus as the region corres- 
ponding to the head of Craniates, the extension of which, 
according to GEGENBAUR, has been determined by that of the 
gill-sliis. The latter conclusion is objected to by VAN WYHE 
(1889, p. 558) who points to the fact that in Amphioxus 
as well as in Elasmobranchs the growth of the branchial 
basket surpasses that of the rest of the body, which causes 
it to extend with increasing age far into the trunk-region 
(cf. p. 41) Thus we see e. g in Elasmobranchs as well 
as in Amphioxus the biliary duct in older embryos recede 
into progressively further backwards situated trunk segments, 
as can be determined by counting. Taking into consideration 
this secondary process, VAN WYHE tries to determine the 
original extension of the branchial basket and thus arrives at a 
number of not much more than nine segments, the same 
number as assumed by him to be contained in the Elasmo- 
branch skull. By the inequality of growth of the branchial bas- 
ket and the rest cf the body, the series of gill-slits afterwards 
extends progressively further into the trunk-region, as far as 
the 27th myotom and perhaps stillfurther in the adult animal. 
In the anterior region of the body the branchiomerism 
originally corresponds to the mesomerism, as shown by 
HATSCHEK (1892, p. 145) and the figures of WILLEY (1891). 
During further growth, however, a “hypermetamerism”’ of 
the gill-slits is established, the number of the latter sur- 
passing by far that of the myotomes and increasing still 
by splitting, until in the adult form it amounts to more 
than 100. It is by no means easy to determine how far the 
eumetamerism reaches and this is rendered still more 
difficult by the asymmetry prevailing in early stages iu thís 
region. Originally there are formed 14 gill-slits of the left 
and 6 of the right side, all situated at the beginning On 
the right side. Afterwards several of the former atrophy, 
leaving only 8 (nrs. 2-9), while those of the right side 
increase to the same number (sometimes 7 or 9). At this 
