394 J. B. JOHNSTON 



of the hippocampus which extends farthest rostrad beneath the 

 corpus callosum (hippocampal flexure). This appears in sec- 

 tions immediately behind the foramen and thus overlaps dor- 

 sally the primordium hippocampi in which the commissure is 

 imbedded. The ventral portion becomes the fimbria proper. 



In the bear (fig. 72) the neuroporic recess projects rostrad 

 between the fornix columns somewhat beyond the rostral border 

 of the anterior commissure. Above the recess the space between 

 the fornix columns is occupied by the hippocampal commissure. 

 Above this are the bundles of the fornix superior and among 

 them a little gray matter which is the continuation of the sub- 

 callosal hippocampus traced back from the level of the genu. 

 At about this level this small mass of gray becomes fused with 

 the primordium hippocampi (septmii). The latter body is here 

 largely filled with the fibers of the fornix system. Its ventricu- 

 lar portion is relatively free from medullated fibers. On the 

 dorsal surface of the corpus callosum is the indusium with the 

 striae Lancisii which will be more fully described in a later 

 section. 



The rat possesses the largest hippocampal formation rostral 

 to the foramen interventriculare that the writer has seen in any 

 mammal. At the olfactory peduncle (fig. 56) a very broad area 

 of deeply staining cortex is seen on the medial surface almost 

 in contact with the olfactory formation. This is reduced to a 

 narrow band at the level of the rostral border of the caudate 

 nucleus (fig. 57). Here is a great development of the deep layer 

 of the tuberculum and between this and the cortex mentioned 

 is a narrow area of small pale cells which represent the primor- 

 dium hippocampi. At the rostral border of the corpus callosum 

 (fig. 58) the special enlargement of the deep layer of the tuber- 

 culum has disappeared and the islands of Calleja invade the 

 lower part of the medial wall. The primordium hippocampi is 

 much increased in size while the band of cortex previously de- 

 scribed is still small. Curving about the genu of the corpus 

 callosum is seen well developed cortex accompanied by the stria 

 medialis Lancisii. Nine sections farther caudad (fig. 59) the two 

 bands of hippocampal cortex have united into a broad band 



