11 



of tlie hi-ain in Cyclostomes is already menlioiicd by Scott ^) in 

 Petroniyzon. 



As stated above, the telescoping is the more obvious in tlio 

 oblongata on account of the reduction in the acustico-Uiteral system 

 of the bulb. 



Everything indeed shows that in Myxine we have to do with 

 considerable secondary modifications. 



Also the topography of the motor nuclei is by no means a primi- 

 tive one. 



The primitive location of the V, VIL and X nuclei in Cyclostomes 

 is near the ventricular ependyma where the matrix of the nerve 

 cells is, and where they are still found in Petromyzon. In Myxine, 

 however, the V — VII nucleus has a ventro-lateral periferal position 

 and the X nucleus a lateral periferal position, a condition that can 

 only be caused by secondary influences originating in the functional 

 i-etlectory relations of this animal. 



The influence which has caused this secondary position is certainly 

 the considerable development of the descending sensory V, which 

 has a dominating influence on the structures of the oblongata, an 

 influence which is the more prevailing since the other seiisory and 

 reflectory paths are either atrophied or poorly developed in this 

 animal. We know that in animals with a well-developed dorsal 

 viscero-sensory nucleus the motor vagal column generally has a 

 dorsal position, adjacent to its sensory grey (Selachians), which is 

 still the case even in Petromyzon. 



On account of these facts we cannot agree with Holm in his 

 statement that Myxine has a more primitive character than Peti'o- 

 myzon. 



Summarizing our results we conclude : 



In Myxine the eye-muscle nuclei arc absent. 



The motor V nucleus is incompletely divided into two parls corre- 

 sponding to the central division of the motor root into two ])arts. 



In the continuation of the caudal V nucleus also the n)ofor VII 

 cells are found, as is also the case in Petromyzon. These nuclei have 

 a ventrolateral position very near the concomitating grey substance 

 of the sensory root. A central V nucleus (Holm) has not been found. 



The posterior viscero-motor column, and also the spino-occipital 

 motor column has shifted considerably frontally. By the adjacency 

 of the earcapsule this shifting coidd only be partly followed, by 

 the motor X roots, which are crowded together on the earcapsula. 



^) Scott. The embryology of Petromyzon. Journal of Morphology Vol. 1, 1887 



