210 THE PINEAL ORGAN 



skate, Raja clavata and the eagle-ray or devil-fish (Myliobatis). In the 

 latter and in Ophidium barbatum (Fig. 153, p. 219) the pedicle of the pineal 

 organ is exceptionally long, the end vesicle reaching to a point considerably 

 in front of the hemispheres of the brain. It is enclosed in a connective 

 tissue capsule which is loosely attached to the membrane which forms the 

 roof of the skull in this situation, there being no groove or pit for the 

 reception of the vesicle. 



In the Holocephali, namely in Chimcera monstrosa and in Callorhynchus, 

 there is a conical recess in the cartilaginous roof of the skull (Fig. 145). 

 This is lined by dura mater which is continuous at the margins of the 

 foramen with the perichondrium on the outer surface of the skull. The 

 recess is filled by a tent-like prolongation of the roof of the diencephalon, 

 which represents the dorsal sac of this region, and is called the " mem- 

 branous pallium " (Fig. 146). The stalk of the pineal organ is attached to 

 the brain in the usual situation between the habenular and the posterior 

 commissures and ascends in front of the midbrain and on the posterior 

 aspect of the dorsal sac to end in a minute vesicle at the apex of the recess 

 in the cranial vault. 



Structure of the Pineal Organ of a Fully Developed 

 Sturgeon (Acipenser) 



As compared with the simple ependymal structure of the pineal 



diverticulum in the early stage of its development (Fig. 147) sections 



through the fully developed organ show that a differentiation has taken 



place into two principal types of cell, namely, supporting cells which 



retain their ependymal character and cells of a sensory type. Two 



other types of cells are also found, but are less frequent ; these are 



large ganglion cells and neuroglia cells. Nerve-fibres and neuroglia 



fibres are also present. The same structure is met with throughout 



the whole extent of the organ, including the proximal part, the stalk, 



and end vesicle. Fig. 148 represents a part of the walls of the proximal 



part of the stalk, stained with iron-hEematoxylin and highly magnified. 



The outer ends of the ependymal cells are attached by a slightly expanded 



foot to the external limiting membrane ; the inner end of each cell is 



slightly notched and lacks any special character (e.g. cilia). The cell 



bodies are palely stained and the cytoplasm has a loose reticular structure ; 



the nucleus also is feebly stained and has a fine nuclear reticulum. The 



cells of the sensory type on the other hand are deeply stained with 



hematoxylin, both as regards the denser cytoplasm and the nucleus ; 



the outer end of the cell is often prolonged into a process which resembles 



a nerve-fibre and runs in a direction parallel to the surface forming a right 



angle with the long axis of the body of the cell. The inner ends of these 



