Johnston, Morphology of the Head. 237 



tracts to the optic lobes. The inference is that the optic tracts 

 are to be compared with the internal arcuate fibers. In support 

 of this inference further facts are obtainable in the fish brain. 

 The first is the intimate relation between the two sets of fibers 

 at their place of ending in the optic lobes. The writer has 

 thought that he could distinguish in Acipenser the sets of cells 

 which served as the center for the optic fibers from those which 

 served as the center for internal arcuate fibers. This distinction 

 is at best a difficult one, can not be extended to the whole of the 

 tectum opticum, and is much more doubtful in Petromyzon (68) 

 and selachians (63). It is to be noticed, however, that the 

 structure which corresponds to the colliculus of higher forms is 

 partly differentiated from the tectum opticum in Acipenser and 

 teleosts and is related to the secondary cutaneous fibers only 

 and not to those of the optic tract. From these facts it must 

 be thought that the tectum opticum was in primitive fishes the 

 common center for the optic tract and the secondary cutaneous 

 tracts and that a differentiation into two centers is in progress in 

 the higher fishes. 



A second fact is found in the examination of the two chief 

 ventral decussations of the brain, the ansulate commissure and 

 the decussations of which the optic chiasma forms a part. 

 Although both of these (especially the latter) require further in- 

 vestigation, this much may be said : both contain secondary and 

 tertiary somatic sensory tracts destined to the same or similar 

 centers. The most important and primitive tracts in these de- 

 cussations are: 



Chiasma and postoptic decussation. Ansulate commissure. 



Secondary: optic tract Tractus bulbo-tectalis (int. arc). 



Tertiary : tractus tecto-lobaris Tractus tecto-bulbaris et lobaris 



? : tractus lobo-bulbaris Tractus lobo-bulbaris 



The correspendence between the second and third elements 

 suggests that the two decussations belong to one commissural 

 system which has been interrupted by the downgrowth of the 

 inferior lobes. Reasons were given in a previous paper (69) for 

 thinking that the inferior lobes are constituted of cells belonging 

 to the category of commissural and tract cells which are found 



