166 MARION HINES 
in the posterior pole and the hippocampus has made the ventro- 
caudal twist into the temporal lobe so characteristic of it. In 
the last two embryos of the series, the 39.1-mm. and 43-mm., 
remarkable growth has taken place in all regions of the neopallium, 
so that relatively little area, comparatively speaking, contains 
the hippocampal formation. And it is worth noting that the 
major portion of the hippocampus in these last two brains lies 
in the medial wall posterior to the velum transversum. Thus trac- 
ing the history of its position in the developing hemisphere lends 
adequate support to a portion, at least, of Herrick’s quadrant 
theory of telencephalic evagination. But, further, this brief his- 
tory of hippocampal position points to the conclusion that its 
extent is inversely proportional to neopallial growth. It also 
gives some facts concerning the regional acceleration of this neo- 
pallial growth, namely, that acceleration seems to shift from the 
frontal to the dorsal and then to the posterior poles of the de- 
veloping hemisphere. 
Concomitant with this change there is the intrinsic differen- 
tiation characteristic of the hippocampus itself. Although set 
aside as the first cortical area, its subsequent differentiation 
progresses so slowly that such layers as are characteristic of the 
cortex appear in the neopallium long before they are completed 
in the hippocampus. 
The differentiation does not proceed in any logical sequence, 
but seems rather to be subject to rhythms of acceleration. These 
rhythms of acceleration do not correspond absolutely to those 
expressed in any arrangement of the adult brains of the verte- 
brate phylum. In other words, given the stage in human de- 
velopment of the hippocampus, the differentiation of the fascia 
dentata or the neopallium will not correspond to the phylogenetic 
development of the first-named tissue. It is possible, however, 
to take any one tissue and follow it through a complete develop- 
ment whose changes fit into its phylogeny. The developing 
neopallium seems to act as a disturbing factor, not, however, as 
one which obliterates, but rather as one which obscures the 
phylogenetic history by suddenly leaping into the foreground 
and by its great increase in amount and complexity of tissue 
