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the Prosencephalon , there being no olfactory lobe , while the nerves 

 themselves agree closely in histological structure and relation with the 

 other cranial nerves. The region thus giving origin to the I. pair of 

 nerves shows markedly the characteristics of a true neuromere. Imme- 

 diately behind the (neuromere I.) are the remnants of the optic diverti- 

 culum. Young Ambly stoma sections show that the brain walls on 

 either side immediately beneath these, are symmetrically constricted 

 thus proving the existence of an optic neuromere with which the 

 II. pair of nerves were probably at one time in connection. This 

 I. have considered the second neuromere. 



A short distance behind it lie the white transversely arranged fibres 

 of the posterior commissure. In this area the Cod brain shows little 

 or no segmentation, but from the fact that it nearly corresponds in 

 extent to neuromere II. and that its existence is quite evident in Am- 

 blystoma, it seems probable that this space is occupied by the III rd and 

 last of the fore-brain neuromeres. 



Mid-brain. 



Cod sections of about 11 or 12 days exhibit a region extending 

 from the posterior commissure to a point about midway between it 

 and the auditory vesicles. Within these limits there appear two well- 

 marked convolutions of the brain wall, which are slightly smaller than 

 those of the fore-brain and rather more semicircular than oval in form. 

 The characteristic radial arrangement of cells is present. From the first 

 in the Cod at a low level some fibres are given off and from the second 

 in Amblystoma, near its dorsal surface some cells are proliferated, but 

 I have not been able to satisfy myself, that they represent the corre- 

 sponding nerves of this region. I feel confident however from the 

 theoretical evidence derived from the character of the fibres of the II. 

 and IV. nerves, that transverse sections and more specific methods of 

 staining will determine more definitely their existence. 



Hind-brain. 



I have little to add to the investigations of McClure for this 

 region. In the Cod 6 neuromeres are to be found corresponding to the 

 number observed by him in the lizard and chick. In Ambly stoma 

 only 5 are found, the VI. or abducent, being absent. In other forms the 

 neuromeres exhibit closely the general characters described by Orr 

 and McClure. 



10* 



