40 



McCLURE. 



[Vol. IV. 



the brain; and that the constrictions of the Myelon 

 gradually pass or merge into those of the Enceph- 

 alon, thereby forming a continuous series of constrictions 

 throughout the entire length of the neuron, which increase in 

 size anteriorly. 



For sake of clearness I have classified the constrictions of the 

 neuron as follows : 



Neuromeres. 



Constrictions of the Myelon = Myelomeres 

 Constrictions of the Encephalon = Encephalomeres 



The number of encephalomeres ^ actually observed in the 

 types examined is as follows : 



Amblystoma . . 

 Anolis and Chick 



HB 



5 

 6 



MB 



FB 



2 

 2 



I do not, with Orr, consider the mid-brain as equivalent to a 

 single encephalomere,^ but rather relying upon the observations 

 of Kupffer, as equivalent to two (or even three) which have de- 

 generated in the above-mentioned forms, but persist in the 

 Teleosts, and probably in other fishes. The total number of 

 encephalomeres was thus probably ten, divided as follows : 



Fore-brain, 2 and possibly a portion of a third. 

 Mid-brain, 2 or 3. 

 Hind-brain, 6 or 5. 



In order to avoid confusion when speaking of the encepha- 

 lomeres individually, I have given them names which I think 

 for the present will answer the purpose. 



II, 



Olfactory IVenroniere. 

 The most anterior neuromere of 

 the primitive fore-brain. 

 Optic Neuromere. 

 The second neuromere of the 

 primitive fore-brain. 



III. Oculomotor A'euromere. 

 Mid-brain neuromere. 



IV. Trochlear Neuromere. 

 Second neuromere of the mid- 

 brain (demonstrated in Petro- 

 7nyzo7i). 



1 Term proposed by Wilder for the large encephalic vesicles which we cannot 

 now consider in any proper sense segmental. See article " Brain " by Wilder in 

 Reference Handbook of the Medical Sciences, Vol. VIII 8., § 23, prop. X., p. 113. 



2 For mid-brain neuromeres, see Appendix. 



