Phenomcna of neurobiotaxis in the optic svstem 281 



In conscquence of this a cooparation of these nuclei ¡s effected of so 

 intímate a character that in some animáis both groups may hardly be 

 distinguished and, in some birds, even partly overlap each othcr. 



Also the frontal shifting of the nucleus trochlearis is repeated in ontogénesis. 

 Bok found in the chick, in vei y early stages of evolution, a still considerable 

 hiatus between the X. trochlearis and the X. oculomotorius. The nucleus trochlea- 

 ris then elongates in a frontal direction (4th day). Only on the i4th day Bok found 

 the hiatus to be filled up, evidently by the approaching of the nucí, trochlearis to 

 the nucí, oculomotorius. Streeter published similar observations with respect 

 to man. 



Also the largest of the eye-muscle nuclei, the nucí, oculomotorius, 

 presents very important differences with regard to its topography and 



Fig. 13. — Xucleus oculomotorius in Petromyzon ñuviatiüs ithe lamprey;. 



the arrangements of its cellgroups, which may be explained by the prin- 

 cipies of neurobiotaxis. 



The nucleus for the innervation of the first headm\"otome, is found in 

 all animáis in the mid-brain. Located frpm the beginning in the centre 

 of the midbrain, it does not exhibit any important frontal or caudal 

 shiftings: but is subject to dorso-ventral ones. In the Lamprey ffig. 13), 

 it consists of two groups of cells which are differently situated. One 

 group, reaching more frontally, holds a completely latero-ventral 

 position cióse to the place where the root leaves the base of the 

 midbrain. (Some authors doubt wether these are rootcells) . 



The chief group of cells reaching more cuadally has a more dorsal 

 position. Its whole structure resembles more or less the condition as 

 shown by the anterior hora of the spinal cord. 



In sharks there are no remnants of a ventral nucleus, the whole com- 

 plex of cells belonging to this nerve lying above and between the coordi- 



