192 



petra. To make this clear I reproduce here (Fig. 9) a figure from my 

 former paper and along with it a drawing of a similar section of the 

 medulla oblongata of the ammocoetes of P. dorsatus. As for the ar- 

 rangement of "amacrine cells", I do not recognize that such cells exist 

 in the brain of Lampetra at least. Examples of all the different types 

 of neurones are found with true axones clearly recognizable. Where 

 axones have not been seen I have attributed it to faulty impregnation. 

 It is true that many axones are difficult to recognize. I have clearly 



v.m.c. 



Fig. 9. A, part of a transverse section of tiie myelencephalon of adult Lampetra 

 at about the level of the N. glossopharyngeus. Reproduced from Johnston 1902 

 (Fig. 9 a). B, left half of a transverse section of the brain of the ammocoetes of Petro- 

 myzon dorsatus at the same level. Both drawings are taken from sections stained by 

 nuclear stains and the nuclei of the cells are drawn under the camera. It is seen 

 that essentially the same arrangement of functional centers is present in both brains. 

 ac. acusticum. Lv. lobus vagi or visceral sensory column, e.g. central gray. IV fourth 

 ventricle, l.m.c. lateral (visceral) motor cotumn. v.m.c. ventral (somatic) motor column. 

 i.a. internal arcuate fibers, sp. V tractus spinalis trigemini. n.sj). V nucleus of the 

 Same (substantia gfelatinosa). 



pointed out that the dendrites are often nearly as slender and smooth 

 as the axones (e. g. in the hypothalamus) and that in many cases the 

 axone takes its origin from some part of the dendrites far removed 

 from the body of the cell. So far as the brain is concerned at least, 



