118 MARION HINES 
woven into the fabric of their development in such a manner 
that later additions of brain substance have not erased the early 
history of the long passage. Consequently, there is a possible 
interpretation of early growth in the human telencephalon based 
upon a comparison of its growth with that attained by various 
representatives of the vertebrate phyla. However, it may ap- 
pear when this study is complete that this so-called biological 
inertia which has been used to explain the striking similarities of 
development in the brains of the vertebrate series has its roots 
not in heredity as such, but in the more fundamental necessity 
of the mechanics of growth. Although certain recapitulations 
are complete, there is no stage of development whose rhythms 
of growth repeat in any way accurately earlier phylogenetic 
stages. If, however, the development of one particular tissue 
is watched with care, the sequence of intrinsic differentiation 
will appear in the order of a phylogenetic ‘recapitulation. It is 
possible that the disturbing element is the growing neopallium, 
whose initial acceleration influences the growth rhythms of the 
other parts of the telencephalon and may therefore have modi- 
fied the early phylogenetic relationships of this tissue. 
DISCUSSION 
Certain factors seem to be implicit in the growth changes of 
the telencephalon. They are the landmarks or points in the 
midline which show individual differentiation. ‘These points are 
delimited by a peculiar histological structure and a characteristic 
external morphology. With such a series of stages as presented, 
the early development in the midline, together with those changes 
in the telencephalic vesicle whose various parts are confluent 
with them, may be followed. 
Telencephalon medium 
The midplane structures of the telencephalon medium have 
been divided into two main divisions by the angulus terminalis, 
the lamina terminalis and the area chorioidea. The lamina 
terminalis grows progressively thicker, rostro-caudally, and the 
thickening approaches the angulus terminalis progressively from 
