468 O. RYLANDS DE BEER 



muudibular somito corresponds undoiibti^dly to a visceral 

 arcli (the first) and a dorsal nerve (trigcuninal) lying behind it. 

 Similarly the ramus ojjhthahnicus profundus is situated 

 posteriorly to the first somite. Again, in the trunk region 

 the ventral root of a somite joins the dorsal root posterior 

 to that somite to form a mixed nerve. Therefore Van Wijhe's 

 scheme involves two discrepancies, viz. that in the regions 

 between the mandibular and hyoid arches and between the 

 posterior somite of the head and the first of the trunk there 

 has been a reversal of the relations between somites and 

 dorsal nerves. The first of these discrepancies concerns the 

 trigeminal and facial nerves. The trigeminal is situated behind 

 the mandibular somite, whereas the facial lies in front of the 

 fourth. To be consistent one would have to attribute the 

 trigeminal to the somite posterior to it (third) and the ramus 

 ophthalmicus profundus to the second, but this would leave 

 the first somite without a corresponding dorsal nerve. Similarly, 

 in the region between the trunk and the head, the last branch 

 of the vagus would lie anterior to its somite (Van Wijhe's 

 ninth), whereas the first spinal ganglion is situated posterior 

 to its somite. Not only would the nerves from the facial to 

 the vagus lie anterior to their somites, but they would also lie 

 anterior to their corresponding ventral roots. In the trunk the 

 ventral root is always more anterior than its corresponding 

 dorsal root (Goodrich, 8). These relations of Van Wijhe's 

 somites are diagrammatically represented in Text-fig. 11. 



We see then that this scheme has to contend with serious 

 difficulties, all of which are the outcome of regarding the 

 third somite as having lost its visceral cleft and arch. Let 

 us now examine this assumption. In the first place the missing 

 gill-slit is not indicated by any of the structures which it must 

 have involved and of which it is reasonable to expect that some 

 vestige would remain. There is no trace of arch, cleft, afferent 

 or efferent blood-vessels or nerve. This in itself is significant 

 in view of the fact that the anterior visceral arches are con- 

 served with constant regularity all through vertebrate phylo- 

 geny. And even when the clefts disappear they leave traces of 



