52 NERVOUS SYSTEM OF VERTEBRATES. 



In the region of the caudal part of the myelencephalon in all 

 vertebrates the neural crest is relatively small. In cyclostomes 



N. trigeminus x 



N. profundus^ 

 Recessus neuroporicus 



Auditory vesicle 

 vii i 



vi ! 



N. facialis. 

 X. glossopharyngeus 

 ' N. vagus 



N. sp. d. i 



N. sp. d. 2 

 N. sp. d. 3 



Optic vesicle ' 



Infundibulum 



A 



Neuromere vi 

 N. profundus Lens 



Optic vesicle 

 Epiphysis '. 



Auditory N. spin. 

 Somite 4 vesicle dors, i sp.d.2 sp.d.3 Somite 10 



' ;sp. d. 5 



Hypophysis 

 and nasal pit 



Praeoral 

 entoderm 



Chorda 



N facialis 



! Tongue muscle bud 



Somite i ' 1 N. glossopharyngeus N. vagus 



N. trigeminus branch, ramus i 



B 



FIG. 28 A. A diagram of the head of Petromyzon at a stage when two gill clefts 

 are open and the neural crest is segmented into the anlages of the cranial ganglia. 

 From the left side. The dorso-lateral placodes are indicated by vertical and hori- 

 zontal cross-lines, the epibranchial placodes by oblique lines. After KoltzofT's 

 diagram VIII. 



FIG. 28 B. A diagram of the head of Petromyzon at a later stage when all the 

 cranial nerves are to be recognized. After Koltzoff's diagram XIII. 



