65 
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 former 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- 
pendance (fig. 17) (cf. p. 273). 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 nerve. 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 representthe 
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 the branchial plexus, according to 
VAN WYHE (1893, p. 162), 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 considerable 
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 
5) 
facialis, glossopharyngeus and vagus, innervate the gill-slit in front 
of their segment, not the one behind it (cf. fig. 17), and t 
Craniates every dorsal spinal root eg with the ventral root following 
p. 99 
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