Johnston, The Brain of Pctromyzoit. 39 



penser which arises from the ciliated cells of the saccus and 

 ends in the thalamus nucleus close to the nucleus of the pos- 

 terior longitudinal fasciculus, I have not found in Petromyzon. 

 Neither have I found the ciliated cells, although my prepara- 

 tions are scarcely fit for the demonstration of these elements. 

 I think it probable that this tract, if present at all, is very 

 small. 



The postoptic decussations are smaller than in any other 

 form which has been studied. They consist entirely (?) of 

 fibers from the cells of the hypothalamus. These go to one 

 of three destinations, epistriatum, cerebellum, or medulla. 



The similarity of the cells of the inferior lobes and corpus 

 mammillare, the origin ot the tracts to the medulla from all 

 parts of the hypothalamus, the common distribution of the de- 

 scending tractus olfacto-lobaris, and the simple character of the 

 postoptic decussations all indicate that the inferior lobes and 

 the corpus mammillare are essentially a unit in Petromyzon. 

 Their functional differentiation as shown by their structiu'e and 

 connections is still slight in Acipenser. The beginning of dif- 

 ferentiation is seen in Petromyzon in the ending of the tractus 

 tecto-lobaris in the inferior lobes alone. 



D. Fore Brain. 



The identification of the several parts of the fore brain 

 must rest solely on the study of the minute structure, on ac- 

 count of the crowding and displacement of the cephalic and 

 lateral portions by the pressure of the great buccal cavity. The 

 growth of this organ has carried the olfactory pit around upon 

 the dorsal surface of the head and the olfactory lobes and areas 

 have been correspondingly bent upward and telescoped back 

 upon the base of the fore brain, the striatum, 

 a. Corpus striatum. 



The two nuclei of which the corpus striatum is composed 

 are much more distinct here than in any other fish. The stria- 

 tum proper forms the base of the fore brain in front of the 

 chiasma and above the preoptic recess, and is continuous 

 laterally, with the ventral wall of the lateral expansions, 



