Johnston, The Brain of Petromyzon. 27 



more freely in the inner part of the tectum and do not have 

 the extravagant end-bushes which are found on the type A cells 

 in Acipenser. The two cells probably belong to the same cate- 

 gory and the reason for the less marked differentiation in Pe- 

 tromyzon is to be found in the fact that the optic tract fibers 

 are not confined to the outer zone as in Acipenser. 



The horizontal cells are much more numerous than the 

 vertical. They either have a single dendrite going off from the 

 peripheral end of the cell and soon dividing into two large 

 branches which are disposed horizontally, i. e. , parallel with the 

 internal surface of the tectum, or they are bipolar with the two 

 dendrites disposed horizontally. Occasionally they are multipolar 

 or stellate. They are larger than the vertical cells, the dendrites 

 have a wide expansion, and these appear to be the most im- 

 portant elements in the tectum. The neurite always arises from 

 some part of the laterally directed dendrite and enters the 

 bundles going through the lateral wall of the mid brain. Some- 

 times the dendrite becomes gradually transformed into a neu- 

 rite after a long course. The unipolar cells are situated near 

 the cavity and there is a progressive modification to the bipolar 

 and stellate forms from within outward. 



In the outer part of the fiber zone there are a few stellate 

 or pyramidal cells with short very richly branched dendrites 

 which are probably to be classed with the pyramidal superfitial 

 cells described below. 



Some of the superficial cells have mitral shaped bodies 

 with two dendrites going off horizontally from the inner end of 

 the cell body. The dendrites run parallel with the outer surface 

 and send their branches somewhat inward. The neurites arise 

 from the laterally directed dendrites, usually by the dendrite 

 changing into a neurite. These cells are to be compared di- 

 rectly with the deep horizontal cells. Occasional horizontal 

 cells in the outer part of the tectum have short neurites 

 (Fig. 12). 



The pyramidal cells (Fig. 13, 25) are somewhat more 

 numerous than those last described and are more sharply char- 

 acterized than any other cells in the tectum. The larger end 



