162 MARION HINES 
primordium and that, as cortical differentiation proceeds in that 
portion which lies anterior to the velum transversum, the hippo- 
campal fissure disappears. But posterior to the velum trans- 
versum the fissura remains as the adult fissura hippocampi 
(fig. 51, sketch D). 
The relation of the hippocampus to the neopallium 
The tissue which manifests the most marked and most regular 
acceleration is the neopallium. In the youngest embryo there 
is no clear line of lateral demarcation of this tissue. But in the 
14-mm. the wall of the ventro-lateral sector is noticeably much 
thickened. This division between the two lateral sectors is more 
marked in the 19.1l-mm., where a slight ventricular groove 
appears. The position of the hippocampus in the developing 
vesicle depends largely upon the amount of neopallium joining the 
hippocampus and the latero-ventral complex. Further, the in- 
trinsic differentiation of the neopallium which is first seen in the 
27.8-mm. progresses more rapidly than that of the hippocampus, 
although that tissue was the first to become at all evident in the 
developing telencephalon. The process of evaginaton is largely 
one of growth between the hippocampus and the region of the 
pyriform lobe, uncus and tail of the caudate nucleus. 
The appended table 5 gives an idea of the relative differen- 
tiation in the vesicle of these various embryos. - 
With these data in hand two factors appear to be involved in 
the position of the developing archipallium. The first of these 
is the disposition of the old cortex in the wall of evagination, 
coincident with the noteworthy acceleration of the neopallium. 
The second factor is the intrinsic differentiation of the hippocam- 
pus itself. From these relationships it 1s possible to delineate 
the method of growth in the evaginating telencephalon. 
Recalling the form of the telencephalon at a stage where 
no cortical area has been evaginated from the telencephalon 
medium into the cerebral hemisphere as exemplified by the case 
of Ichthyomyzon concolor already cited (Herrick and Oben- 
chain, ’13, figs. 3 and 4), it is evident that in the process of further 
