THE FISSURA HIPPOCAMPI 79 
This is the first primordium of the hippocampus. The thickening 
of the wall is limited exactly to the middle part of the groove while the 
wall retains its original thickness at the transition in the choroid fis- 
sure and in the outer mantle, the inner edge forms in cross section a 
stronger band than the outer . . . . (p. 280). The fissura am- 
monis is not the result of a gradual infolding, but rather is to be con- 
sidered a secondary formation in the outer layer of the thickened wall 
; (p. 281). If the series is followed through one finds that 
the fold, if we can give the groove this name, is more accentuated in 
the posteri ior portion than in “the anterior. A division into two origins, 
an anterior and a posterior, as His described in the human embryo 
(89), I have not been able to confirm (p. 282). 
The history of the fissuration on the medial wall of the tel- 
encephalon of man, during the second, third, and fourth months, 
falls naturally into the following subdivisions: 
1. Fissuration is an artefact and therefore of no morphological 
significance. 
2. Fissuration is not an artefact. It is accompanied by charac- 
teristic histological structure: a, without future significance; 
6, with future significance. 
If these contradictory statements are true, there is a pos- 
sible resolution. The solution found rests almost entirely upon 
a consideration of histological structure and a correlation of 
development of the tissue in question. To follow the region which 
lies in the fissura arcuata of His from its earliest differentiation 
as a tissue distinct from the remainder of the vesicle to its ulti- 
mate destiny is the purpose of the present paper. This analysis 
will give the first point of departure in studying the development 
of the forebrain as a growing tissue with the hope that at some 
future time the interrelationship of its various parts may be 
expressed mathematically. 
During the progress of this research I have sought constantly 
the aid and advice of Dr. G. W. Bartelmez, and depended largely 
upon the manifold suggestion and the critical judgment of Dr. 
C. Judson Herrick. Without them it would have been impossible 
to begin or carry to completion this piece of work. Iam happy 
also to acknowledge the debt I owe Dr. R. R. Bensley, not 
for aid in this particular problem, but for the scientific training 
I possess. Further, I wish to acknowledge the use of material 
