FROM TWELVE TO THIRTY-SIX SOMITES 



153 



infimdibulum (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): 



5tOJ77. 



Oes. - 



Jyy?e/)c 



Am. 



Fig. 87. — Optical longitudinal section of the head of an embryo of 30 s. 

 The heart is represented entire. 

 Atr., Atrium (auricles). B. a., Bulbus arteriosus. D. v., Ductus venosus. 

 Lg., Laryngo-tracheal groove. Oes., Oesophagus, or. pi., Oral plate, which 

 has begun to rupture. Parenc, Parencephalon. Ph., Pharynx. Stom., 

 Stomach. Synenc, Synencephalon. Th., Thyroid. S. 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- 

 35 s stage the epiphysis (pineal body) begins to form as an 

 evagination from about the middle, and by the 36 s 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 



