Johnston, Morphology of the Head. 193 



tion of nerve roots connected with the hind brain. This will 

 be discussed below (Sec. 9). 



The segments lying in front of the cerebellum are vari- 

 ously interpreted owing to the comparatively early appearance 

 of a segmentation corresponding to the permanent brain vesi- 

 cles. The segmentation of the brain tube in very early stages, 

 before the formation of these vesicles, has been studied by 

 ZiMMERMANN ( 1 24, rabbit, chick, selachians), Waters (ii8, 

 teleost), LocY (125, selachians, with supplementry observations 

 on the chick and amphibia), and Hill (52, teleost and chick). 

 All these authors studied the neuromeres in all regions of the 

 brain before the closure of the neural tube, in which stage 

 those of the hind brain had already been seen by Kupffer and 

 Froriep ; and Locy and Hill have traced the continuous his- 

 tory of the neuromeres to the time of their merging into the 

 permanent brain vesicles. The work of these last two authors 

 is evidently most painstaking and their results are so complete 

 and so far in agreement that they may be taken to represent 

 the present state of knowledge of the neuromeres. They agree in 

 assigning three neuromeres to the fore-brain and two to the mid- 

 brain. The writer may say that at first he was strongly inclined 

 against the conclusions of these authors on account of the un- 

 expectedly large number of segments recognized in the anterior 

 part of the brain. However, he has lately made some observa- 

 tions on amphibian embryos {Amblystoma pjuictatwn), repeating 

 and extending those made by Locy, and although these obser- 

 vations are not yet complete they seem thus far to confirm 

 Locy's work. Further, it will appear in the course of this pa- 

 per that the point of view from which the subject is approached 

 leads to the acceptance of the number of neuromeres described 

 by Locy, as necessary to account for all the nerves and sense 

 organs connected with the brain. 



Neal (96, 97) has studied the neural segmentation in sela- 

 chian embryos and does not accept Locy's interpretation of the 

 early segments in the fore part of the brain as true neuromeres. 

 He has called especial attention to the existence of a blank 

 neuromere between Nn. V and VH and his description enables 



