396 John F. Holm 



The caudal sensory cranial nerves, as the N. Lateralis and Vagus, 

 have one rostral and one medial root nucleus. 



This Hypoglossal nerve has the rostral nucleus differentiated 

 from the sensory region of the central gray, while the medial one 

 is still undifferentiated. The sensory nerves in Myxine are an illus- 

 tration of how the cranial nerves gradually develope out of the 

 simple spinal nerves, without independent root nuclei. We have 

 the Hypoglossus with one root nuclei and the Vagus and Lateralis 

 with two, then the Acusticus and Trigeminus with their relatively 

 complicated conditions, the root nuclei of which are distributed over 

 three quarters of the whole of the Oblongata. 



It also shows how the development of special sense organs in 

 the rostral part of the body and the increasing importance of the 

 nerves have transformed the morphological conditions of the spinal 

 cord into the more complicated ones of the Medulla oblongata. It 

 is remarkable that this transformation is entirely due to the sensory 

 elements, as the nuclei of motor elements of the Oblongata are in 

 no way bigger than that part of the ventral column which may be 

 considered to belong to each spinal nerve, the division into separate 

 motor nuclei in the Oblongata being caused by the increase the 

 Oblongata obtained in all dimensions through the development of 

 the sensory elements. 



Another question is, the general condition of the brain of Myxine, 

 whether it may be considered to be in a state of degeneracy or in 

 a primitive or embryological state. To this question the answer 

 can be given both ways, but chiefly in favour of the latter hypothesis. 



No doubt the low state of the ocular apparatus and especially 

 the absence of the oculomotor nerve may be taken as signs of de- 

 generacy, though it is doubtful whether the ocular apparatus in this 

 animal has ever performed the functions of a vertebrate eye. On 

 the other hand, the arrangement of the remnants of the ventricular 

 system, the absence of any signs of a Cerebellum and the primitive 

 state of the Oblongata indicate that we have here a vertebrate brain 

 which stands on a lower stadium than any other known except that 

 of Amphioxus, a state so low that it may perhaps be compared to 

 other fish brains in less than half their embryological development. 



