104 MARION HINES 
tioning its reality? You may well do so, when its history is 
remembered. Certain it is that the question of fixation was 
adequately controlled. The embryos studied were carefully pre- 
served, their further handling as good as our present technique 
allows, and yet such treatment did not banish from the medial 
wall of the early telencephalon the fissura arcuata of His (the 
fissura hippocampi of others). Nevertheless the description of 
this fissure seems to indicate that its preterminal limb is transient. 
If this groove is real, there must be some explanation, first for its 
appearance and second for its partial obliteration. Further, 
if it is real, a histological differentiation would be expected and 
processes of growth differences may account, in part at least, 
for its appearance and later its disappearance in the prevelar 
region. It is not a question of fixation. A very meager ex- 
perience with this material gives an unerring criterion to the 
investigator for the determination of artefacts. The explanation 
is to be found, rather, in the development not only of the tissue 
involved in the fissure itself, but also in the histological differ- 
entiation of tissue in more remote parts of the developing hemis- 
phere wall. 
HISTOLOGICAL STRUCTURE 
The 11.8-mm. embryo, Mall Collection, 1121 (figs. 21 to 25) 
The telencephalic vesicle of the brain of this embryo is little 
more than a single evagination, which has expanded slightly 
beyond its initial attachment to the diencephalon. ‘The lateral 
expansion of this vesicle is greater in the region midway between 
its anterior and posterior poles. ‘The midline extending from the 
region of the velum transversum rostrally is interrupted by an 
infinitesimal elevation, the paraphyseal arch. Figures 21 to 
25 are line drawings of sections of the vesicle cut at right angles 
to the chord joining the velum transversum and the preoptic 
recess. The levels are indicated in figures 8 and 11. The mid- 
line, beginning in the region of the bed of the optic chiasma, may 
be divided morphologically into the following regions (figs. 7 
and 8 and p. 85 supra): 
Region of the optic chiasma (fig. 21, F. b. op. ch.). 
The lamina terminalis pars crassa (fig. 22, Lam. term.). 
