( 36 ) 



almost exclusively round or pear-shaped mesenc. quintus cells; the 

 blniit-cornered elements of the hydrosaurii are entirely or nearly 

 entirely, lacking. The distribution of the cells in sagittal dimension 

 of the nucleus is somewhat different viz. alligator sclerops shows 

 the greatest development of the nucleus in the frontal ]jart of the 

 tectum (fig. 13), eunectes murinus in the most distal part (fig. 14;, 

 varanus saloaior (which in this respect does not widely differ from 

 chelone midas) possesses mesencephalic roof-cells disti-ibuted fairly 

 evenlj' over the tectum opt. (fig. 15). The point of exit of the mid- 

 brain root is in principle the same in all the reptiles examined. 

 It was clearest in my preparations of varanus salvator and boa 

 constrictor. The respective fibres of varanus begin to send a portion 

 towards the pei-iphery only when the sens, trigemenus is already 

 making its entrance. A large part of the spinal quintus root has 

 already been formed, and e\ en lies fully developed in the oblongata, 

 when the out-going mesenc. V fibres reacii the periphery, or rather 

 the ventral part of the rad. spinalis V. Ventrally from this and 

 separated from it, lies the out-going mot. trigeminus root (fig. 16). 



]LSihs.4es( 



Fig. 16. Varanus salvator. 



In the other reptiles the conditions are the same; only in the 

 case of boa did I succeed in showing with any certainty a remnant 

 of the fibres which run with the mesenc. V root proceeding in a 

 caudal direction, on the medio-dorsal boundary of the subst. gelat. 

 Rolando rad. spinalis Vi. 



Birds. The extraordinary development of the tectum opticum, its 

 lateral-ventral bending and the thinning of the lamina commissuralis 

 bring about the lateral position of the mesenc. quintus cells above 

 the ventr. lob optici, as is shown by fig. 17 ; only in the most oral 



