( 345 ) 



elements is it composed at its first appearance? The third question 

 of Rktzius lias been amplified by Zuckehkandl ') and may be formu- 

 lated as follows: what are the changes occurring in the mesial wall 

 of the pallium in consequence of the development of the corpus 

 callosum? 



For the answering of the first question a human embryo of the 

 beginning of the fourth month was at my disposal. The fronlo- 

 occipital diameter of the corpus callosum amounted to but 0.5 in. in. 

 Figure 1 shows a frontal section through the more posterior part of 

 the lamina terminalis. 



The plane of section deviating a little from the frontal plane, that 

 which is shown in the right part of the drawing is more frontally 

 placed than that which is shown in the left part. 



As appears from the drawing the corpus callosum lies in the 

 lamina terminalis; especially on the left this is clearly evident, where 

 the undernorder of the pallium goes over into a taenia (7') which 

 is bent in and passes over into the lamina terminalis {Lit.). The fact 

 that the ependyma of the lamina terminalis, which is continued into 

 the ependyma of the taenia, also spreads underneath the corpus 

 callosum, obviates all doubt as to the existence of this relation. It' 

 now the sections are examined more frontally, it will be seen, that 

 the more frontal part of the corpus callosum no longer lies in the 

 lamina terminalis. This part of the corpus callosum exceeds the 

 limits of the lamina and is situated in the zone of union of the 

 mesial walls of the pallium. This zone is built up of glia-tissue and 

 in immediate continuity with the glia-layer covering the fore-side of 

 the lamina terminalis. 



On the ground of this observation 1 believe that the corpus cal- 

 losum originates in the lamina terminalis, very soon, however, in 

 consequence of the enlargement of the commissure, preponderantly 

 in a frontal direction, it encroaches on the lamina and lies partially 

 in the zone of union of the pallia. 



Another embryo, of the middle of the fourth month, exhibits a 

 corpus callosum with a maximum diameter of 2.5 m.m. Here the 

 commissure is still entirely situated in front of the foramen Monroi. 

 Figure II shows a frontal section through the more posterior part 

 of the corpus callosum. In this section the corpus callosum {C.c.) 

 lies most dorsally, laterally going over into the mesial wall of the 

 pallium. In this wall, aside from the callosum, we tind the fornix 



') Zuckerka.ndl. Sil/.b. lv. Acad, der W. Math. Naturw. el. Ltd. CX. h. \ 

 Wieii 1901. p. 234. 



