ORIGIN AND STRUCTURE OF THE HE/vD 131 



to the myotome behind it, as is the case in Craniates ^). 

 In front of the mandibular somite, however, there are two 

 pairs of nerves which mainly contribute to the innervation 

 of the snout, a weaker anterior pair and a stronger second 

 pair, the foimer situated more ventrally. the latter more 

 dorsally. They are compared by HatsCHEK (1892) with 

 the two portions of the trigeminus of Craniates which as 

 a rule is considered as a double nerve but which we view 

 as a single one. part of which has acquired a certain inde- 

 pendence (fig. 17) (cf. p. 87). Indeed, on looking at fig. 5 

 it becomes at once apparent that in front of the mandibular 

 somite there is room for but one single i.erve. If we accept 

 the view of HATSCHEK, to which also the application of my 

 theory leads, then the dorsal parts of the next three segmental 

 nerves in Amphioxus can only represent the facialis, the glos- 

 sopharyngeus and the primary vagus (fig 26, B, C, D), while 

 the next three would be the dorsal roots of the hypoglossus, 

 from which in higher Craniates the accessorius is to be deriv- 

 ed. Is it by chance that in the figure of HATSCHEK the fourth 

 nerve (the fifth of the authors ', which would then represent the 

 vagus (fig 26, D), is so considerably thicker than the ones in 

 front of and behind it? Is there any relation between it and the 

 longitudinal plexus supplying the gill-slits? According to 

 HATSCHEK a strong visceral nerve branch (fig. 26 NR) goes 

 from this fifth nerve to the branchial plexus, according to 

 VAN Wyhe ( 1 893, p. 1 52), however, it only supplies the velum. 

 These questions must evidently await further investigation. 

 Neuropore and praeoral lobe in Ascidians. — Until now 

 we have left the Ascidians out of consideration. While we 

 must assume that Amphioxus has undergone consideiable 

 reduction, the structure of the Ascidian-larvae and Appen- 

 dicularia evidently shows so great deviations from the 

 original plan, which is so easily recognizable in early stages 

 of development in Amphioxus, that it is hardly possible to 

 reconstruct the common ancestor of the Urochordates and 

 Cephalochordates from the structure or development of 

 the former. Does the neuropore and the brain-vesicle of 



^) The four dorsal head-nerves in Craniates e.g., the trigeminus, 

 facialis, glossopharyngeus and vagus, innervate the gill-slit in front 

 of their segment, not ihe one behind it (cf. fig. 17), and in higher 

 Craniates every dorsal spinal root unites with the ventral root following 

 behind it (cf. Hatschek, 1893, p. 9j). 



