I96 THE PINEAL ORGAN 



small, caudally directed diverticulum, the wall of which is formed by a 

 single layer of cells. This soon becomes bent forward, and after having 

 considerably increased in size two parts may be distinguished, a proximal 



Fig. 138. 

 A and B : Median sagittal sections of the parietal region of the brain, showing early 



stages in the development of the pineal organs of Petromyzon. (A after 



Kuppfer ; B after Dohrn.) 

 C — Transverse section of the rudiments of the pineal organs of a 37-day-old 



embryo of Salmo fontinalis. (After Hill.) 



ch. : habenular commissure. 



cp. : posterior commissure. 



/. po. : left pineal organ (anterior). 



/5. ; lamina supraneuroporica. 



MB. : roof of midbrain. 



po. : pineal organ. 



pp. : parapineal organ (Kuppfer and Studnicka). 



r. po. : right pineal organ (posterior). 



st. : common stalk of right and left pineal diverticula. 



hollow stalk immediately in front of the posterior commissure, which has 

 meanwhile been developed, and a dorsoventrally flattened vesicular end- 

 part which lies over the stalk, namely the " terminal vesicle " (Fig. 139). 

 The terminal vesicle at this stage of development lies directly over the 

 ganglia habenulae, which are already present, and it raises the skin over it 



