THE PINEAL SYSTEM OF FISHES 207 



spinal cord. At later stages in development a certain degree of differen- 

 tiation takes place, into neuroglia cells and cells which resemble ganglion 

 cells. Neuroglial fibres and nerve-fibres also may be distinguished, 

 the latter being most abundant near the base of the stalk, where they 

 become collected together in bundles. A definite pineal tract is present 

 in the posterior part of the stalk near its base, which according to Studnicka 

 is traceable into the posterior commissure ; other fibres have occasionally 

 been traced into the habenular commissure and ganglia, e.g. in Acipenser. 

 The structure of the proximal part of the pineal organ is essentially the 

 same as that of the stalk. In some specimens, however, e.g. Spinax, the 

 wall may be considerably thickened and thrown into folds. Folding of 

 the wall is also frequent in the end vesicle, but this is not so common as in 

 the bony fishes, in some species of which it is so pronounced that the organ 

 somewhat resembles a racemose tubular gland, more especially as the 

 spaces between the folds are occupied by vascular connective tissue 

 (Fig. 155, p. 220). In the cartilaginous fishes the wall is usually only 

 slightly folded or is smooth. It is covered by a thin sheath of pia mater and 

 accompanied by vessels which form a plexus around the terminal vesicle. 

 Pigment is frequently present in the connective tissue around these 

 blood-vessels, but not in the walls of the vesicle itself. 



The General Form and Position of the End-vesicle of 

 Cartilaginous Fishes 



The end vesicle of these fishes, as compared with that of Cyclostomes 

 and certain reptiles, is relatively small, and in some cases it is probable 

 that it simply represents the slightly dilated distal extremity or blind end 

 of the stalk — the true sensory vesicle not having been developed, or if 

 developed in early embryonic life having been cut off from the stalk and 

 subsequently degenerated, as appears to be the case in some amphibians. 

 There are, however, certain points with regard to its shape, structure, and 

 situation that seem to favour the view that it does in reality represent the 

 sensory vesicle, in the majority of cases. In no instance, however, does 

 it reach its full development, but remains small and imperfectly differen- 

 tiated. With reference to its shape, certain cases occur in which the end- 

 organ shows evidence of bilaterality : it may be heart-shaped, having a 

 distinct notch on its anterior border, e.g. Raja clavata and Acanthias 

 vulgaris (Cattie) ; or it may be mallet-shaped, forming with the distal 

 extremity of the stalk a T-shaped junction which projects beyond the 

 hemispheres, e.g. Centrophorus granulosus. Moreover, the stalk is fre- 

 quently accompanied by two principaf arteries which spring from the right 

 and left sides of the arterial anastomosis which surrounds its base. In many 

 •cases, however, the end vesicle has been described as conical, club-shaped, 



