296 THE PINEAL ORGAN 



it is possible that in some cases the lobules may have arisen as solid out- 

 growths. In the adult the main body of the organ is often separated from 

 the roof of the brain by rupture of the thinned-out stalk. It lies in the 

 narrow triangular space between the cerebellum and the two hemispheres. 

 This relation in the adult bird contrasts markedly with the embryonic 

 condition in which there is a wide interval between the cerebellum and 

 the pineal outgrowth, which space is occupied by the prominent optic 

 lobes. The space is reduced in the later stages of embryonic development 

 by the straightening out of the cephalic flexure of the brain and the 

 relatively greater growth of the hemispheres and cerebellum as com- 

 pared with the optic lobes. 



Hem 



Pi C hob 

 V C post 

 Fig. 203. — The Parietal Region of the Thalamencephalon and the Epiphysis 

 of an Adult Fowl (Gallus domesticus), showing the Almost Solid 

 Condition of the Follicles and Ingrowth of Connective Tissue. (After 

 Studnicka, 1905.) 



Cbl. : cerebellum. Ep. : epiphysis. 



C. hab. : habenular commissure. Hem. : hemisphere. 



C. post. : posterior commissure. St. : stalk. 



Ds. : dorsal sac. 



The shape of the epiphysis in the adult is usually either spindle- 

 shaped or clavate, and it is directed almost vertically upward towards 

 the roof of the skull. The solid type of epiphysis may be seen in the 

 adult fowl, but different grades exist between the embryonic follicular 

 form (Figs. 196, 197) and the almost solid form sometimes seen in adult 

 examples. 



Between these three principal types of epiphysis there are all degrees 

 of intermediate forms, and different parts of the organ in a single individual 

 may conform to the character of any of the three types. In some examples 

 there are evident signs of degeneration, such as the disappearance of 



