206 THE PINEAL ORGAN 



has come on insidiously, and has taken up a position behind the vesicles of 

 the paired eyes, in apparently the same position previously occupied by the 

 accessory vesicles. These transpositions are confusing, as they resemble 

 the accessory optic vesicles grown larger." It will be unnecessary to 

 enter into details of the discussion which followed the publication of 

 Locy's work. This was chiefly concerned with the position of the 

 accessory vesicles relative to the lateral optic vesicles and the supposed 

 homology between the accessory optic vesicles and the median paired 

 eyes of arthropods ; and if Locy's conception that " there are preserved 

 on the cephalic plate of certain Elasmobranch embryos three pairs of optic 

 vesicles " — the lateral and two accessory pairs — were true, a condition 

 would be present in the embryo of these fishes which is well known in 

 certain invertebrates, and it would provide a basis for the supposition that 

 the ancestors of the vertebrates possessed two or more pairs of eyes ar- 

 ranged serially and that one pair, the lateral eyes, have evolved to form the 

 highly differentiated and inverted lateral pair of eyes of living vertebrates ; 

 whereas the other accessory pair (or pairs) have degenerated, or possibly 

 have evolved along a different structural plan and functional direction, 

 and have persisted as the pineal organ. Locy's conception of the ultimate 

 fate of the depressions A. op.' ; A. op.", and their connection with the pineal 

 diverticulum, has however not been generally accepted by later authors. 



Structure of the Pineal Organ of Selachia ' 



The walls of the end-vesicle and stalk are throughout their whole 

 extent built up on a foundation of ependymal cells which extend between 

 the inner and outer limiting membranes. The oval nuclei of these 

 cells are situated at varying distances from the internal limiting membrane. 

 The ends of the cells where they are attached to the membrane are often 

 expanded, as is the case in the supporting radial fibres of Miiller in the 

 retina of the lateral eyes of vertebrates. These conical expansions are 

 most noticeable at the outer ends of the cells. At the inner end some 

 of the cells terminate flush with the membrane with which they are 

 blended. Others send small club-shaped or irregularly tapering processes 

 through the membrane, which project into the lumen of the tube. These 

 processes are seen both in the terminal vesicle and in the stalk. No 

 definite ciliated epithelium is, however, present. In many cases the 

 lining membrane is quite smooth, though in others thread-like proto- 

 plasmic processes project inwards which end in a nucleated syncytial 

 network, like the " vitreous " of the end-vesicles in Petromyzon. In 

 some cases there is a coagulum resembling an inspissated secretion, such 

 as is frequently seen in the ventricular cavities and central canal of the 



1 Cartilaginous fishes belonging to the sub-order of Elasmobranchii which includes 

 the Sharks and Dog-fishes. 



