192 Journal of Comparative Neurology and. Psychology. 



rangement has led to the most various results. Since the brain 

 is a single continuous organ, the study of its segmentation 

 would appear to be a comparatively simple matter. It has long 

 been known that the spinal cord and medulla oblongata show a 

 segmentation in early embryos which corresponds to the meso- 

 dermic segmentation. Indeed, the segments of the cord and 

 brain were early regarded as merely the effect of mechanical 

 pressure from the mesodermic segments. It has been clearly 

 shown, however, that the transverse divisions are true segments, 

 due to a definite arrangement of elements in the brain wall and 

 often appearing before the mesodermic segmentation. 



In spite of the apparent simplicity of the case, the study 

 of the brain has led to scarcely better agreement as to the 7ium- 

 ber of segments than the study of the cranial nerves. A sum- 

 mary of the findings of various workers is given in the papers 

 by LocY (125) and Neal (97). In the hind brain, where the 

 neuromeres are relativel}' distinct, they have been studied by 

 numerous authors and in all classes of vertebrates. The dis- 

 agreement among authors is due to the lack of a definite cau- 

 dal limit to the brain and to differences in fixing upon the first 

 neuromere to be assigned to the hind brain. The differences 

 <:an be harmonized, however, by reference to the position of 

 the nerve roots and ear, and by considering the shifting of nerve 

 roots which has taken place. This discussion properly belongs 

 to a future section (Sec. 9), but certain facts may be stated 

 here. Accepting Hoffmann's enumeration which later work 

 shows to be correct, rather than that of Okr and McClure, 

 there are in the hind brain of reptiles (Orr, Hoffmann, Mc- 

 Clure), birds (McClure, Hill), and mammals (Zimmermann) 

 seven neuromeres ; with the 2d, 4th, 6th and 7th of these are 

 connected the Nn. V, VII-VHI, IX and X respectively. There 

 are thus two neuromeres, one between the Nn. V and VII and 

 one between the Nn. VII and IX, whichare without nerve roots. 

 For convenience such neuromeres will be referred to in the fol- 

 lowing pages as "blank" neuromeres. In fishes and amphibia 

 there is but one blank neuromere, that between Nn. V and 

 VII, and there is less agreement among authors as to the posi- 



