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 



Auditory vesicle 



X. facialis. 

 X. glossopharyngeus 

 I i N. vagus 



N. sp. d. 



X. trigeminus^ 

 X. profundus ^ 

 Recessus neuroporicus 



Recessus 

 praeopticus 



Optic vesicle 



Infundibuluni 



Neuromere vi • Auditory X. spin. 



N. profundus Lens ; Somite 4 \ i-^ick- ilors. i sp.d.2 sp.d.3 Somite 10 



Optic vesicle 

 Epiphysis ; 



Hypophysis 

 and nasal pit 



Praeoral i 

 entoderm ' 



Somite i 



X facialis • ! i 



[ Tongue muscle bud 



X'^. glossopharyngeus ^'- vagus 

 X. 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 Koltzoff'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. 



