250 Journal of Coniparative Neurology and Psychology. 



nection with the ciUary nerve (79) as the epibranchial placode 

 of the fourth segment. 



b. Segment 3. 



Neuromere jii, most anterior portion of the median entoderm (?), epiphysis i. 



Neuromere iii represents the thalamus but according to 

 Hill's results (52) does not include the hypothalamus which is 

 developed in teleosts as a ventro-caudal expansion of the lower 

 wall of neuromere ii. No mesoderm and no motor nerves are 

 present but within the brain is the thalamic nucleus of origin of 

 the somatic motor fasciculus. Its location here possibly indi- 

 cates the former presence of a motor nerve in this segment; no 

 other explanation occurs to the writer. The representative of 

 the general cutaneous nerve is found in the anterior epiphysis. 

 The innervation of the skin is taken over by the profundus. 

 As suggested above, the central nuclei belonging to the cu- 

 taneous division have probably become reduced to an insignifi- 

 cant remnant on account of the degeneration of the epiphysis. 

 The ganglia habenulae in their structure resemble the central 

 grey and they may be regarded as one of the earliest special 

 centers to be differentiated from it. The suggestion has been 

 made earlier (69) that the central grey belongs to the category 

 of the commissural and tract neurones which are found in the 

 oblongata and cord. The reason for the large development of 

 these central elements in the diencephalon is to be found in the 

 reduction of the somatic and visceral sensory and visceral motor 

 centers. The posterior commissure, whose nucleus of origin 

 lies in this neuromere, is spoken of below (Sec. 17). 



c. Segment 2. 

 Neuromere ii, lateral eyes, hypophysis. 



The interpretation of the eyes as the representative of the 

 cutaneous division has been given above. In the central nerv- 

 ous system important centers are developed. Neuromere ii in- 

 cludes the striatum area and also that of the pallium. The 

 writer has previously referred to the striatum as probably repre- 

 senting- the tract cells of the cord and oblongata. The nature 



