Functional factors in the mor[)hology of the forebrain of fishes 153 



instructive. The details of internal structure of this species have not been 

 vvorked out. They are drawn for the most part from Johnston's accounst 

 of related species of petromyzonts. 



The forebrain of the petromyzonts ífig. 7j differs from the generaHzed 

 pattern presented by Acipenser (fig. l) in the very important respect, 

 that the evaginated cerebral hemisphere contains the large olfactory bulb 

 and in addition a part of the secondary olfactory área termed the olfactory 

 lobe which seems to be comparable, in a general way, with the área 

 olfactoria lateralis of other vertebrates. The remainder of the telen- 

 cephalon is retained in the unevaginated telencephalon médium in form 



Fig. 7. — Diagrammatic longitudinal section through the forebrain of the lam- 

 prev, Petromyzon, illustrating the form of the cerebral hemispheres. The hemis- 

 pheric evagination includes a large olfactory bulb and a smaller olfactory lobe, 

 leaving a considerable residue of tissue in the unevaginated primitive endbrain, 

 or telencephalon médium. Conventional markings as in figure i. 



not unlike that of Acipenser (fig. 8), though apparently in more primitive 

 arrangement. 



From the bulbus olfactorius ñbers of the tractus olfactorius stream 

 backward into the área olfactoria, some directly into the área olfactoria 

 lateralis which forms the posterior pole of the evaginated hemisphere, 

 some below the foramen interventriculare into the área olfactoria media- 

 lis, and some above the foramen into the large área olfactoria dorsalis, 

 the last two lying in the telencephalon médium. There is a part of the 

 telencephalon médium below and behind the foramen which does not, so 

 far as yet described, receive any fibers of the tractus olfactorius. 



Extending forward from the hypothalamus is a column ot cells, com- 

 prising the nucleus preopticus (nucleus taeniae of johnston, iq02) and 

 área olfactoria medialis. This column of cells is nearly but not entirely 

 interrupted by the enormous chiasma ridge. The área medialis is reach- 



