WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 47 



For the purpose of comparing the impregnation of the tectal region in 

 these Pleuronectidse with that of the same region' in a symmetrical fish, 

 in order to ascertain whether there are any noticeable histological dif- 

 ferences, I have applied the Golgi method to the brain of Fundulus 

 heteroclitus, the mud minnow. These were found to take the stain very 

 much more easily than do flounders; but there was also more of the 

 silver precipitate carried inward from the surface. I conclude, there- 

 fore, that the tissue in Fundulus must be more open. Except as to the 

 size of certain cells and the relative thickness of some fibre bundles, 

 the two brains correspond closely. The cells of the nidulus corticalis 

 in the minnow are much smaller proportionately, though their tectal 

 processes can be followed in layers 3 and 4 as far as in the Pleuro- 

 nectidse. The spindle-shaped cell found most abundantly in layer 4 

 was again in the minnow the most noticeable cell impregnated, and was 

 found most often. A triangular cell in layer 5, very similar to the 

 cell o found in the corresponding layer of the flatfish, had its outward 

 process extended to layer 1, where it fibrillated like an ependymal cell. 



Most of the cells of layers 3, 4, and 5 in Fundulus had neurites 

 traceable into layer 5, the fillet layer. 



VI. Theoretical Considerations. 



The conditions in the tectum are the same as those found in the optic 

 lobes of typical Teleostei. The division of the tectum into layers is of 

 importance as a means of more precise description. There must be a 

 place where the fibres of the optic tract, which come in as layer 2, end ; 

 that region is layer 3. There must be an association system connecting 

 with the posterior motor regions, and the fibres of this system are either 

 a part or the whole of layer 5. If only a part, then the purpose of the 

 commissura mesencephali is to put the two optic lobes in communica- 

 tion with each other. The cells in layers 3, 4, and 6, especially the 

 spindle cells in layers 3 and 4, probably serve to receive and transmit 

 optic stimuli. 



The nidulus corticalis, developing early, as it does, is probably one of 

 the most effective association centres of the brain.' Lying at the entrance 

 to the tectum, with a strong bundle of neurites running through the two 

 niduli rotundi in the ventral part of the brain, and with its numerous 

 large dendrites passing into layers 3 and 4 of the tectum, it should be 

 able to connect the optic sensory region with the motor areas quickly, 

 and thus account for the extreme rapidity of movement of these larvae. 



