ANATOMY OF A CYCLOSTOME BRAIN 649 



still farther dorsally is the eminentia fimbriae. The latter re- 

 sembles the habenula in structure and is traversed laterally by 

 fibers from the primordium hippocampi to the habenula and me- 

 dially by fibers of the tractus olfacto-habenularis from the nu- 

 cleus preopticus and nucleus olfactorius medialis (p. 660.) 



The commissural ridge which contains the optic chiasma and 

 the postoptic commissure complex (see beyond) is enormously 

 developed in Ichthyomyzon, and is here termed the 'chiasma 

 ridge.' Between the chiasma ridge and preoptic recess below 

 and the interventricular foramen above is a flat triangular pre- 

 optic area of the ventricular wall which contains the preoptic 

 nucleus, the medial olfactory nucleus and the primordium of the 

 corpus striatum. The apex of this triangle lies above the pos- 

 terior end of the chiasma ridge and its base is formed by the 

 lamina terminalis and anterior commissure. 



The cross sections show a compact thick layer of small cells 

 which crosses the medial plane bordering both the upper and the 

 lower surface of the chiasma (figs. 5, 6). Both of these cell plates 

 probably correspond with the preoptic nucleus of fishes and am- 

 phibians, the massive nucleus having been separated into two 

 parts by the backward growth of the commissural ridge contain- 

 ing the optic chiasma and the postoptic commissure. The ven- 

 tral plate which will here be termed the postoptic nucleus, is 

 continuous below with the central gray of the hypothalamus whose 

 cells are more diffusely arranged (figs. 6, 7). 



The cell plate lying dorsally of the chiasma ridge, in the tri- 

 angular area above referred to, likewise consists of a thick densely 

 crowded collection of cells adjacent to the chiasma (the preoptic 

 nucleus) and a more complex dorsal portion. The latter is sep- 

 arately differentiated only in the rostral three fourths of the tri- 

 angular preoptic area. 



In the dorsal part of this area, forming the ventral wall of the 

 sulcus medius and the interventricular foramen, is the primor- 

 dium of the corpus striatum. This is a well defined plate of 

 rather large loosely arranged cells which form the floor of the 

 foramen for a considerable distance laterally in the hemisphere 

 (figs. 3,4, 5,6, C.S.). 



