OEIGIN OF THE COEPUS CALLOSUM. 55 



As we proceed forward in tlie examination of coronal sections, the dorsal commissure 

 will be found to become gradually thinner and finally disappear altogether, while the 

 desceudmg foniix-fibres remain unchanged. The ventral commissure disappears in 

 about the same coronal plane. The " commissw^e-hed " extends a slight distance further 

 forward before the hemisplieres become separated one from another. 



Pigure 7 represents (about four times the natural size) the appearance of a coronal 

 section immediately in front of the place where the hemispheres become separated from 

 each other. 



In this figure the hippocampus is again easily recognizable, although its conformation 

 has become greatly simplified. The fissura Idppocampl (f.h.) is much shallower, and the 

 hipj)Ocampal projection into the ventricle less prominent. The fascia dentata is much 

 narrower, and consequently much less folded. The thin alveus may be seen proceeding 

 directly into the mass of desceudinfj forwix-fihres which occupy the superficial portions 

 of the grey mass which we may call the " corpus prcecominissiiralc" *, but which is really 

 only the anterior continuation and homologue of the grey mass which further back has 

 been called " commisstire-bed.'" The other features of the section do not concern us in 

 considering the hij)pocampal formation. The only other point to which I wish to call 

 attention is tliat the " corpus prcecommissurale " is directly continuous below the lateral 

 ventricle with the corpus striatum, where the latter is capped by the peculiar cortex 

 which Ganser calls the " cortex of the head of the corpus striatum," and which 

 corresponds to the " tubei'culnm olfactorimi " [t.o.) — such a prominent feature upon the 

 ventral aspect of this brain. 



If we were to trace these various structures forward in a series of coronal sections, 

 we should find that the hippocampal fissure would become gradually shallower until it 

 eventually disappeared, the prominence in the ventricle disappearing pari passu. The 

 fascia dentata, now placed entirely upon the surface, becomes narrower and less clearly 

 differentiated, until at last we fail to recognize any distinguishing feature of a hippo- 

 campus. But by the time this has occurred we have arrived almost at the olfactory 

 pedvtncle. In certain Marsupial brains {Dasyuriis clverrinus) it is even possible to 

 distinguish the hippocampal formation as far forward as the olfactory peduncle, so that 

 here wo have the "precommissural area " completely cut off from the general cortex or 

 pallium {p.) by a hippocampal band, as semi-schematically represented in figure 8 

 {hip." & f.d.). This is the primitive condition of the hippocampus, which is found 

 (although recognized with difficulty) in the Reptile, and which I have already described 

 in the fcetal OrnithorMjnclms (Qu. Join-. Micr. Sci. vol. xxxix. pp. 181-206). 



Pigure 9 represents the appearance which this region presents in Perameles when 

 viewed from its mesial aspect. 



In this figure the optic nerve (opt) is seen extending towards the lamina terminaHs, at 

 the inferior extremity of which is the oval section of the optic tract (o.tr.). Extending 

 upward from the optic tract is the thin ventral portion of the lamina terminalis' — 

 the lamina cinerea {I.e.). At its upper part this lamina expands to enclose the large 



* By " corpus pnicommissunde " is meant that grey mass the mesial surface of which is the "precommissural 

 area " (p.a.). 



SECOND SERIES.— ZOOLOGY, VOL. VII. 9 



