52 McCLURE. [Vol. IV. 



the hind-brain, which, less differentiation and specialization 

 having taken place, naturally conforms more to the primitive 

 vertebrate type. I am aware that the forms examined are in- 

 sufficient to enable us to reach any positive conclusion in regard 

 to the exact number of segments, but I feel confident that the 

 method which I have adopted is the one by which this vexed 

 question of the primitive segmentation of the head region, both 

 of the neural tube and indirectly of the surrounding mesoblast, 

 will eventually be decided. 



In conclusion, I may say that I feel confident that the full 

 number of primitive encephalomeres will be found in Elasmo- 

 branch, Ganoid, or Teleost embryos, the investigation of which 

 will form the second part of this paper. 



APPENDIX. 



The Mid-brain Neuromeres. 



Just before sending this paper to the press my attention was 

 called by Dr. Osborn to an article published in the Journal 

 of Morphology by Dr. W. B. Scott, on the Embryology of 

 Petromyzon, in which two distinct neuromeres are figured in 

 the mid-brain (Figs. lo, ii). Dr. Scott makes no mention of 

 these structures as having any segmental value, and in Fig. 1 1 

 the cell structure shown is evidently purely schematic. My 

 Figs. lo and 1 1 are taken from Dr. Scott's plates. The gradual 

 transition of the hind-brain neuromeres into those of the mid- 

 brain is clearly shown in Fig. lo. The gradual transition of 

 the myelomere into the neuromere of the hind-brain in the 

 Newt, Lizard, and Chick has already been mentioned. Thus I 

 think that an examination of various stages of Petromyzon 

 embryos will show a continuous series of neuromeres through- 

 out the entire length of the neuron. 



BIBLIOGRAPHY. 



1. Ahlborn. Ueber die Segmentation des Wirbelthier-Kbrpers. Gbttingen, 



4 Januar, 1884. 



2. Beard. The System of Branchial Sense-Organs and their Associated 



Ganglia in Ichthyopsida. Studies from the Biological Laboratories of 

 the Owens College. Vol. I. 1886. 



