MORPHOLOGY OF THE FOREBRAIN 461 



and its floor is occupied by an undifferentiated primordium hip- 

 pocampi, filled with fimbria fibers and containing scattered large 

 cells. Comparison with embryonic and adult turtles indicates 

 that the position of the nucleus lateralis septi laterally of the 

 primordium hippocampi is secondarily acquired. The nucleus 

 lateralis arises wholly ventral to the primordium hippocampi and 

 grows upward into its present position late in the ontogeny. 



The columna fornicis passes downward rostral to the anterior 

 commissure to enter the medial forebrain tract for the most part 

 farther forward than the plane of this section. The tract desig- 

 nated fornix (c.f.) here contains also external arcuate fibers, i.e., 

 axons of the cells of the nucleus lateralis septi. 



Fig. 67 passes through the rostral border of the anterior and 

 hippocampal commissures and the caudal end of the nucleus 

 lateralis septi, above which lies the primordium hippocampi 

 containing scattered cells and fimbria fibers. Associated with 

 the commissura hippocampi is a nucleus of densely crowded cells, 

 which is clearly homologous with the nucleus of the commissura 

 hippocampi described for Amphibia. Here it lies ventrally of 

 the commissural fibers and just caudad of the level figured its 

 cells cross the median plane. Cells of the precommissural body 

 surround this commissure, the anterior commissure and the for- 

 nix and are mingled amongst their fibers, but this nucleus is 

 sharply circumscribed and easily distinguishable from the pre- 

 commissural body. 



At this level the precommissural body is bounded above by the 

 recessus superior and below by the preoptic nucleus. It dis- 

 appears immediately caudad and there is no pars fimbrialis ex- 

 tending backward above the interventricular foramina. The 

 tract marked fornix (c.f.) in this figure contains external arcuate 

 fibers, columna fornicis and tractus cortico-habenularis. 



At the level of the foramina (fig. 68) the nucleus preoptic us is 

 greatly enlarged and large fiber tracts pass between it and the 

 corpus striatum complex. 



The nucleus of the hippocampal commissure is in contact later- 

 ally with a similar nucleus among the fibers of the tractus cortico r 

 habenularis as they pass upward and laterally from the fornix 



THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 5. 



