FROM TWELVE TO THIRTY-SIX SOMITES 1ot 
primary fore-brain; but, owing to the simultaneous lateral expan- 
sion of the telencephalon, the line of separation in the lateral 
walls forms a curve with the convexity directed posteriorly 
(Figs. 83 E and 86). 
(c) The next stage in the differentiation of the telencephalon 
(20 s to 36s) is characterized by a rapid expansion and evagina- 
tion of its lateral walls, while the entire median strip extending 
from the velum transversum to the recessus opticus remains prac- 
Fic. 85. — Optical sagittal section of the head of an embryo of 22-23 s. 
The heart is represented entire. 
Atr., Atrium. Hyp., Hypophysis. Inf., Infundibulum. Md., Man- 
dibular arch. or. pl., Oral plate.  Pr’o. G., Preoral gut. Th., First in- 
dication of thyroid. T. p., Tuberculum  posterius. V. tr., Velum 
transversum. Other abbreviations as before 
tically unaltered; and thus 
the surface between these 
form a pair of outgrowths 
anteriorly, and posteriorly 
acts like a rigid band stretched over 
two points. The effect of this is to 
that soon begin to project dorsally, 
(Fig. 83 E); these are the primordia 
of the cerebral hemispheres, the cavities of which thus appear 
as lateral diverticula of the median cavity of the telencephalon 
(Fig. 86). The central part of the telencephalon may be called 
the telencephalon medium, and the lateral outgrowths the hemi- 
spheres. The walls of the hemispheres become considerably 
thicker in this period, but quite uniformly at first, so that the 
distinetion between mantle and basal ganglia is indicated only 
(See Chap. VIII.) 
by position. 
