SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 397 



is seen that the paUial commissure lies in a lamina supraneuro- 

 porica containing gray matter and that the tela is attached to 

 the dorso-rostral border of this, forming the recessus superior. 

 The projection of the chief volume of the lamina supraneuro- 

 porica into the ventricle is a characteristic difference between 

 marsupials and higher mammals. In the bat (fig. 45) an inter- 

 mediate condition is found. The pallial commissure has already 

 the crescent shape which is characteristic of those forms in which 

 the corpus callosum has begun to separate from the hippocampal 

 commissure. In the caudal thick part of the commissure are 

 found a number of small bundles of non-medullated or lightly 

 medullated fibers and the rostral part of the crescent seems to 

 contain many lightly medullated fibers. This rostral horn of the 

 crescent of course forms the precommissural portion of the alveus. 

 Whether the lightly medullated fibers have different functions 

 from the heavily medullated can not be decided at present. In 

 the section lateral to the median plane it is seen that the pri- 

 mordium hippocampi is directly continuous with the fascia den- 

 tata through the commissure bundles (figs. 33, 34). The cells 

 of the fascia dentata with which the primordium comes into 

 relation are those in the concavity of the fascia dentata which 

 Kolliker ('96, p. 737, fig. 777) and Edinger ('04, p. 333 and fig. 

 232) call the end-portion of the layer of pyramidal cells of the 

 hippocampus. In transverse sections this continuity of the pri- 

 mordium with the hippocampal formation is not conspicuous 

 because the alveus seems everywhere to form a complete par- 

 tition of fibers between the two gray masses. In sagittal sections 

 it is seen at once that the alveus is in bundles between which 

 columns of cells connect the two masses ; or, rather, that the two 

 form one continuous mass which is traversed by the alveus 

 bundles. 



When transverse sections stained by a cell stain are studied 

 more carefully instructive facts are brought out. First, as is 

 well known from the work of Elliot Smith and Levi, the fascia 

 dentata in its rostral part is directly continuous with the ven- 

 tral border of the hippocampal cortex, as seen in a transverse 

 section (fig. 28). In both hippocampus and fascia dentata is 



