118 THE ANCESTRY OF VERTEBRATES 



Vertebrates we must first consider yet another question, viz. 

 that of the 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 considers 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. 

 Thus we see e. g. in Elasmobranchs as well as in Am- 

 phioxus the biliary duct in older embryos recede into pro- 

 gressively further backwards situated trunk segments, as can 

 be determined by counting, (cf. p. HI) Taking into consider- 

 ation 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 of the body, the series of gill-slits afterwards 

 extends progressively further into the trunk-region, as far as 

 the 27th myotom and perhaps still further 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 WlLLEY (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 in this 

 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 



