No. 3-] THE VERTEBRATE HEAD. 54 1 



with those of the hind-brain. I have taken this to signify that 

 the hind-brain has encroached on the territory of the spinal 

 cord, and has embraced at least one segment originally belong- 

 ing to the cord. It is reasonable, on theoretical grounds, to 

 suppose that such a process has taken place and has been 

 several times repeated. 



In order to bring the segments of the brain into satisfactory 

 evidence I have completely laid bare the brain-walls by removing 

 the overlying layers of mesoderm and epidermis. The segments 

 after being exposed in this way stand out so clearly that I feel 

 considerable confidence in my count of them in Squalus acan- 

 thias. In addition to the nine segments in the hind-brain, I 

 have found five in the combined fore- and mid-brain. In the 

 latter particular I agree with Waters, but, of course, differ 

 as regards the total number in the brain. McClure says (p. 39) : 

 " I will show that Dr. Hoffmann is probably wrong in con- 

 sidering the hind-brain as consisting of seven segments, and 

 that the segment considered by him as the first segment of 

 the hind-brain is rather the posterior segment of the mid-brain ; 

 in other words, it is the second neuromere of the mid-brain." 

 The recently adduced evidence is all in favor of Hoffmann's 

 observation. According to my observations, there are still 

 represented in the otogeny of Squalus at least fourteen paired 

 neural segments belonging to the brain region. There should 

 be added to this enumeration the median unsegmented tip 

 which terminates the line of segments in front, and, as stated 

 above, there are reasons to suspect (although no direct evidence 

 in that structure) that there may be more than one segment 

 included in that terminal piece. If the terminal piece repre- 

 sents a single pair consolidated, then there are fifteen neural 

 segments in the earliest condition of the brain of Acanthias. 



7. Relation of the Neuromeres to Sense-Organs and Cranial 



Nerves. 



The sense-organs and cranial nerves, undoubtedly, at first 

 sustained definite segmental relations to the neural segments. 

 In the spinal cord there is still a pair of nerves for each neuro- 



