ii CEANIAL NEEVES 123 



In the first place we find in the head region as in the trunk a 

 tendency for the nervr-librcs to cnino oil' from the central nervous 

 system in segmentally arranged clumps, and for the motor fibres to 

 be situated more ventrally and the sensory more dorsally. In the 

 head region however the dorsal root becomes reinforced by a large 

 mass of motor fibres which have become shifted dorsalwards and 

 incorporated with it. 



A neural crest develops resembling that of the trunk and in the 

 Birds it can be seen similarly to have a paired origin, arising before 

 the complete closure of the medullary tube. This neural crest of the 

 brain region forms an anterior prolongation of that in the trunk : it 

 is quite continuous with the latter, it develops outgrowths similarly, 

 and the intervening portions here also persist for a time as a longi- 

 tudinal commissure. A number of the most important cranial nerves 



pan 



B? 



FIG. 68. Acanthias, stage 23, 9 mm. long, showing ganglia of cranial and spinal nerves. 

 (After Scammon, 1911.) 



int, intestine ; I, lens ; li, liver ; pan, pancreas ; sp.g, ganglia of spinal nerves ; Th, thyroid ; 

 V, ventricle ; v.c, visceral cleft ; y.st, yolk-stalk ; IV, V, etc., ganglia of cranial nerves. 



are simply prolongations of the outgrowths in question V, VII, 

 VIII, IX and X. 



A conspicuous feature in the development of the cranial nerves is 

 that in portions of their length they receive components directly from 

 localized thickenings (placodes) of the ectoderm (Kupffer, Beard) a 

 possible reminiscence of the time when nerve-trunks became evolved 

 out of <a plexus in direct relation to the external ectoderm. 



I. The Olfactory nerve is unrivalled amongst all the sensory 

 nerves of the Vertebrates as a subject for investigation on account 

 of its large size, its short uncomplicated course and its retention of 

 comparatively primitive conditions even in the adult. Eesearch 

 should therefore be specially concentrated upon its mode of 

 development. 



In the case of Lepidosiren, as already indicated, Elliot Smith has 

 shown that the olfactory nerve is simply a drawn - out primary 

 connexion between brain and olfactory organ, already present at a 

 period before these organs have begun to move apart. 



In other vertebrates (Elasniobranchs Holm, Tek-osts, Amphibians 



