FROM TWELVE TO THIRTY-SIX SOMITES 158 
infundibulum (Fig. 83 E); thus the diencephalon may be divided 
into an anterior and posterior division, parencephalon and synen- 
cephalon (Kupffer) (Fig. 87). The optic stalks are attached to 
the floor and ventral zones at the extreme anterior end. The 
diencephalon includes part of the roof, floor, and dorsal and ven- 
tral lateral zones of the original neural tube. These may be de- 
scribed as follows (Figs. 87 and 88): 
Myelenc. 
MeESENC. 
— Zp. 
7 Symenc 
FParerne 
Fig. 87. — Optical longitudinal section of the head of an embryo of 30s. 
The heart is represented entire. 
Atr., Atrium (auricles). B.a., Bulbus arteriosus. D. v., Ductus venosus. 
Lg., Laryngo-tracheal groove. Oeces., Oesophagus. or. pl., Oral plate, which 
has begun to rupture. Parenc., Parencephalon. Ph., Pharynx. Stom., 
Stomach. Synene., Synencephalon. Th., Thyroid. 8S. v., Sinus venosus. 
Ven. R., Right ventricle. Other abbreviations as before. 
The roof rises quite sharply from the velum transversum, and 
is indented between the parencephalic and synencephalic divi- 
sions as already noticed. It is relatively thin. About the 30- 
35s stage the epiphysis (pineal body) begins to form as an 
evagination from about the middle, and by the 36s stage is a 
small hemispherical protuberance (Figs. 86 and 88). The floor 
becomes extremely thin in the center of the recessus opticus, which 
marks its anterior end; immediately behind this is a sudden and 
