THE PINEAL EYES OF CYCLOSTOMES 



197 



so as to form a slight prominence. The whole organ at this period is, 

 relatively to the brain, exceptionally large ; its transverse diameter being 

 little less than that of the interbrain. The terminal vesicle which at first 

 had a large lumen becomes transformed into a " loaf-like " structure. Its 

 upper wall remains thin and single layered, whilst its lower wall becomes 

 thickened, its superficial surface being arched upwards so that the floor 

 and roof of the vesicle are in close contact with each other, and in transverse 

 section the lumen appears as a crescentic cleft, convex superficially 

 (Fig. 139). After a time, however, the lumen again enlarges and the two 

 walls of the vesicle become separated again. Still later (Fig. 137, B), the 

 lumen of the stalk disappears, except below where a small part persists as 

 the recessus pinealis, which lies in front of the posterior commissure, and 



Fig. 139. — Transverse Section through the Pineal Organ of a 6-mm. Embryo 

 of Petromyzon planeri. (After Studnicka.) 

 Epd. : epidermis. po. : pineal organ. 



hg. : habenular ganglion. st. : stalk. 



above in the situation where the stalk becomes continuous with the terminal 

 vesicle. From the lower wall or retina of the terminal vesicle nerve- 

 fibres grow into the stalk, the cells of the latter being pushed aside or 

 penetrated in the same way as in the development of optic nerves of the 

 paired [lateral] eyes, and it is in this way that the nervus pinealis is 

 developed. At the upper end of the stalk, there is formed at a later stage a 

 fairly large hollow cavity, the " atrium," the walls of which " remind one " 

 of the structure of the retina, and as a rule the atrium opens into the 

 cavity of the end vesicle near the centre of the retina, where there is a 

 funnel-shaped recess. Subsequently the parietal organ is shifted forward 

 from its point of origin in the region of the ganglia habenulae to a position, 

 in the fully developed brain, in the region of the hemispheres. 



Studnicka's description of the development of the parapineal organ 

 is not so explicit and it is probable that the recess which is shown in 

 Kuppfer's diagram in front of the anterior commissure does not give origin 

 to the parapineal organ, but that this arises as a subdivision of the primary 

 pineal diverticulum. 



From the foregoing description it will be evident that the pineal 



