84 THE ANCESTRY OF VERTEBRATES 



oblique backward course, this incongruity appears to be-, 

 much less distinct in very young stages where the gill-slits 

 have just broken through, it is precisely the oblique course 

 of the posterior head nerves, as argued already by 

 GEGENBAUR (1872, p. 253, 275), which is a strong indi- 

 cation of the original congruence between the cranium and 

 the series of gill-slits, however much the two may differ 

 in backward extension in the adult. The curve in backward 

 direction made by the ventral nerves of the cervico-brachial 

 plexus round the last gill-slits to reach the ventral side of 

 the body, must be considered equally as a result of the 

 secondary backward expansion of the series of gill-slits, 

 which in young embryos appears to be still much less 

 pronounced. We come then to the conclusio;i that GEGENBAUR. 

 was not far from the truth in assuming that the posterior 

 limit of the Elasmobranch cranium coincides with the back- 

 ward extension of the gill-slits, though, as we shall see further 

 on, we may not go so far as to assume that the number of 

 segments incorporated into the cranium may be deduced 

 from the number of visceral archs. 



As to the second part of the third question it need not 

 again be emphasized that the branchial region does not 

 belong to the primarily unsegmented anterior part of the 

 head, the prostomium, but to the segmented soma. 



Thus on the whole my theory leads to a confirmation 

 of the views of GEGENBAUR, VAN Wyhe and ZlEGLER 

 with his disciples, and to a rejection of those of FRORlEP 

 in so far as his conception of the branchial region as an 

 unsegmented head region is concerned. 



Significance of praemandibular cavity. — As we have seen, 

 the praemandibular segment, at first described by BALFOUR 

 (1878), is considered by VAN WYHE and ZlEGLER as the 

 first segment of the soma. To this end the N. trigeminus 

 must be considered as a double nerve, the ramus ophthal- 

 micus profundus with the ciliary ganglion representing the 

 dorsal root to the first segment, while the oculomotorius 

 is taken as the ventral root. As a matter of fact the latter 

 innervates exactly those four eye-muscles which are produ- 

 ced by the praemandibular segment, which lies closely^ 

 applied to the eye-ball. Thus everything required for a 

 segment is present with the exception, however, of one- 

 thing: the division into a dorsal and a ventral part. Only 

 the former is present, the latter is missing, and with it the 



