SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 401 



Werkman's photographs of sagittal sections of this region in bat 

 embryos are extremely instructive in this connection CWerkman 

 '13, figs. 13, 17 and 18). These show the relations between the 

 paraterminal body and the septum in three stages. Essentially 

 the same condition is seen in the embryos of the rabbit, cat and 

 other mammals. If now we turn to the selachian and compare 

 Werkman's figure 18 and figures 45 and 55 of the present paper 

 with the median section of Scyllium (fig. 84) we find an almost 

 complete correspondence of part for part. The external neuro- 

 poric recess in the selachians is a deep canal containing blood 

 vessels. In some selachians the canal is obliterated and the 

 vessels are imbedded in a solid mass of tissue. An external pit 

 is retained in the adult bat and mole and in all embryos. In 

 the bat and mole a cell-free and fiber-free band occupied by 

 blood vessels marks the position of the external neuroporic recess. 

 The same is true but less prominent in some other mammals. 

 In the human embryo of 31 mm. the external neuroporic recess 

 is penetrated by a special group of blood vessels (fig. 8). The 

 mole and the bat show in a striking manner that even when 

 there is a large corpus callosum, as in the mole, the essential 

 relations of the area parolfactoria and the hippocampal forma- 

 tion with its pallial commissures do not differ at all in selachians 

 and mammals. 



One other point in the brain of the mole may be mentioned 

 here. When the region of the splenium is studied in transverse 

 sections it is seen that the enlargement of the indusium behind 

 and beneath the splenium (fasciola cinerea) consists of much 

 larger cells than those in the adjacent hippocampus proper (fig. 

 54). These cells form a plate which is continuous at its lateral 

 edge with the layer of pyramids of the hippocampus. The fascia 

 dentata takes no part whatever in the formation of the indusium. 

 As the hippocampus is followed forward beneath the corpus 

 callosum the cells of the fasciola cinerea gradually blend to some 

 extent with those of the hippocampus. This plate of cells is, 

 however, readily followed forward beneath the hippocampal com- 

 missure until the fascia dentata disappears from the section and 

 this plate of cells merges with the septum pellucidum as above 

 described (fig. 53). 



