1039 



Tlie trigeminus nucleus has the same dorsal position and length 

 as in Acipenser and Amia. Its sagittal topography recalls rather 

 that in Acipenser than in Amia. 



The abdocens leaves the same between the VI 1 and IX roots 

 with four rootlets as was the case in Amia. 



The location of its nucleus could not be stated with exactness, 

 the position of the cells being too diffuse amongst the reticular cells 

 of that region. It resembles, however, the position in the other Ganoids 

 in so far as its cells do not form two well delined groups as occurs 

 in bony fishes, nor do thev have such a ventral position as in 

 Teleosts. The |)OSterior viscero-motoi- column has the same position 

 as in the other ganoids. Its frontal limit is nearly the same, the 

 caudal extension seems a little shorter, which may be due to the 

 young stage of development. 



It contains the motor VII, IX, and X nuclei, but the cells are not 

 equally large every where : groups of large cells alteinate with groups 

 of smaller cells, of which the motor character is not so conspicuous. 

 It may be that this means a little discontinuity in this motor column. 

 It does not however give us sufficient evidence to speak of isolation 

 of different nuclei. 



The position of the spino-occipital rootlets and cells resembles 

 very much that found in Amia. 



On an average it may be said that Lepidosteus shows principally 

 the same type in the an-angement of its motor roots and nuclei as 

 the two other Ganoids. 



A few words may be added concerning a structure at the base 

 of the medulla oblongata near the spino-occipital rootlets : the 7mc/<^wó' 

 paramedianus or oUva inferior. 



In this region the dark aspect of the tegmental part of the bulb 

 changes for a lighter one in the Weigert-Pal preparations, owing loan 

 enlargement of the grey matter consisting of small, more or less 

 spindle-shaped cells and a sort of "gelatinous" substance. The caudal 

 and frontal limits of this structure are not sharp, but the bulk of it 

 extends in the places indicated at the base of my topographic schemes 

 by little crosses. 



From this structure a crossed myelinated fibre tract runs along 

 the lateral border of the oblongata to the region where the cerebel- 

 lum joins the bulb. Also Johnston (1. c. p. 16) describes such crossed 

 fibrae arcuatae externae. 



The character of this crossed cerebellar connection, the sort of 

 cells that constitute the nucleus paramedianus and its topographic, 

 relation, prove that we have to consider it as a pi'imitive oliva inferior. 



Go 



Proceedings Royal Acad. Ams^loidam. Vol. XVI. 



