398 J. B. JOHNSTON 



seen a sparse layer of polymorphous cells next to the ventricle, 

 in part mingled with alveus fibers. In the rostral part of the 

 fascia dentata this ventricular layer consists of a much larger 

 number of cells and these are broadly continuous with the mass 

 of the primordium hippocampi (figs. 28, 32). Further caudally, 

 where the fascia dentata becomes displaced so as to embrace 

 the border of the hippocampus in its concavity, the cells which 

 fill this concavity are seen in favorable sections in direct con- 

 tinuity with the primordiiun through the alveus (figs. 29, 30, 

 31). From these facts it appears that the polymorphous layer 

 contains the primitive cells of the hippocampus and is com- 

 parable to the primordium hippocampi. This is especially clear 

 in the relations of the primordium to the hippocampal cortex 

 in the pre-callosal region in the rat (figs. 58, 59). 



The brain of the bat is rather small for dissection and the 

 writer has not had a sufficient number of specimens to warrant 

 using them in this way. Therefore the median plane of this 

 brain is reconstructed from sagittal sections and a parasagittal 

 section is drawn to illustrate the arrangement of centers in the 

 medial wall (figs. 45, 46). These figures show clearly that while 

 the nucleus parolfactorius medialis extends high up toward the 

 pallial commissures it does not reach them but these commis- 

 sures are imbedded in a considerable mass of gray matter which 

 lies dorsal to the level of the neuroporic recess. This mass is 

 the primordium hippocampi and is separated from the area par- 

 olfactoria below by a well marked cell-free zona limitans hippo- 

 campi. The primordium consists of two parts, a dense collec- 

 tion of small cells forming a bed for the hippocampal commissure 

 (fig. 46), and a much less dense area of larger cells beneath 

 the corpus callosum. Rostrad the zona limitans rises almost 

 to meet the genu of the corpus callosum, but in sections to one 

 side of the median plane (fig. 47) the supra-callosal hippocampus 

 (indusium) surrounds the rostral border of the commissure and 

 becomes continuous with this sub-callosal portion of the pri- 

 mordium hippocampi. Rostral to the genu the hippocampal 

 formation extends to the olfactory peduncle. Close to and in 

 the median plane the large celled portion of the primordium is 

 reduced to small size but is distinct from the nucleus parolfac- 



