THE FISSURA HIPPOCAMPI 163 
evagination the most dorsal edge of the massive side wall will 
become the most ventral edge of the complete evagination and 
help form the roof of the foramen Monroi(p.126). Anterior to the 
lamina terminalis the most dorsal border will meet the most 
ventral edge, making a seam or junction along the medial wall 
of the growing telencephalon. 
This seam or junctional zone on the medial wall of the cerebral 
hemisphere in front of the lamina terminalis is always marked 
in amphibian and reptilian brains by a cell-free limiting zone and 
often by a ventricular groove, a sulcus limitans hippocampi. 
This area is necessarily transitional in type because, besides 
the approximation of the hippocampal formation with the septum 
(the primitive dorsal column with the primitive ventral column), 
it marks the union of the thinner dorsal part of the telencephalic 
roof plate bordering the hippocampal formation with the septum, 
the major portion of which sooner or later becomes greatly 
thickened. The sulcus limitans hippocampi marks the border 
of the hippocampal formation for its entire length, and rostral 
to the foramen interventriculare it also marks the junction of 
the hippocampal formation with the septal complex. 
In this series the cerebral hemispheres of the 11.8-mm. embryo 
have reached what may be called the first stage in the evagina- 
tion; the primordial hippocampus is dorsal throughout and there 
is no medial wall anterior to the lamina terminalis. The hippo- 
campal formation lies as a crescent over the top of this evagina- 
tion, never quite reaching either the anterior or the posterior 
pole of the vesicle, yet lateral and dorsal to the sulcus limitans 
hippocampi. In the 14-mm. the hippocampus is entirely medial, 
anterior to the angulus terminalis, and climbs, as it were, over 
the crest of the hemisphere to the posterior pole. Here again 
the differentiation only approaches the posterior pole, but does 
not reach it. The amount of neopallium is greater in this embryo 
at the anterior pole than at the posterior. This process continues, 
so that the formation in the 19.1-mm. and the 20-mm. lies en- 
tirely upon the medial wall. In the latter embryo, however, 
the neopallial tissue in the posterior pole has increased. In 
the 27.8-mm. and the 32.1-mm. the neopallium has grown greatly 
