404 W. H. GASKELL. 



disappeared in consequence of the closure of its walls by the 

 lateral compression of the nervous material. 



On the one hand, then, this particular specimen shows what 

 is only indicated in others, that the remarkable ventral projec- 

 tion known by the name of the conus post-commissuralis 

 is due to the growth of nervous material around a ventral 

 median tube, which arises from the cephalic stomach near its 

 pyloric orifice ; on the other hand, it shows that the closure of 

 this tube has taken place in such a manner as to form a 

 vertical slit by the compression of the nervous material on 

 each side, just as in the case of the formation of the raphe, 

 and that the tube comes to the surface of the brain in a very 

 similar manner to the tube which forms the old oesophagus 

 already described in a former section. In both cases the 

 structure of the actual end of the tube is a reticulum filled 

 with transparent material, so that in the fresh condition the 

 end of the conus post-commissuralis and the infundi- 

 bular projection are so transparent as to allow the central 

 cavity to be dimly seen through their walls. 



The nervous material immediately around the raphe is 

 similar to that surrounding the ependyma, and is composed 

 therefore mainly of the small berry-like cells already spoken 

 of; some of these cells continue onwards around the ependyma 

 of this ventral tube or duct, as it may be called, and so in com- 

 bination with the broken-down epithelium of the duct walls 

 gives rise to the structure known as the ganglion inter- 

 peduncular e. The connection of the ganglion interpedun- 

 culare with the cells of the central grey matter is noticed by 

 Ahlborn, Avho says (3, p. 254) " es ist nicht zu bezweifeln, 

 dass die Zellen des Ganglion interpedunculare thatsachlich aus 

 dem centralen Bodengrau hervorgegangen sind.^' 



We see, then, that the extraordinary shape of the brain on 

 the ventral side, with its two projections in front of and behind 

 the large ventral constriction, is due to the presence in the 

 middle line of two tubes, both of which pass from the central 

 cavity to the surface : the one tube, compressed to form a 

 horizontal slit, leads out of the third ventricle, i. e. out of the 



