23O THE PINEAL ORGAN 



the posterior commissure and the pineal tract passes backwards on the 

 dorsal aspect of the epiphysis to end in the ventral part of the posterior 

 commissure. 



The end vesicle or parietal organ is usually a round, hollow, epithelial 

 cyst, but is sometimes solid. In the former the superficial or distal 

 wall of the cyst is formed of a single layer of epithelium, while the proximal 

 wall shows two or three layers of nuclei, which in Bufo, according to 

 Studnicka, are radially arranged, and may contain pigment granules. 

 The end vesicle of Bufo thus shows a more primitive (less degenerate) 

 condition than in Rana, and resembles somewhat the end vesicle of Petro- 

 myzon, but is not so highly differentiated. The cavity of the end vesicle 

 varies considerably in size and shape. In some cases it is irregularly 

 constricted and has the form of the figure eight placed horizontally 00 

 (R. esculenta) ; or kidney-shaped (Bombinator igneus) (de Graaf) ; or 

 horse-shoe shaped (Leydig). It is sometimes quite irregular in form and 

 crossed by protoplasmic strands. The whole vesicle is enclosed in a 

 loose connective tissue capsule composed of flattened cells, between which 

 and the vesicle is a space which Leydig (1891) has shown in Bombinator is 

 traversed by nerve-fibres and capillary vessels. The superficial surface 

 of the vesicle is formed by a layer of flattened neuro-epithelium (external 

 limiting membrane), which constitutes the inner boundary of the space. 

 As a rule the epithelial cells forming the walls of the end vesicle remain 

 undifferentiated, but in some specimens of adult animals Studnicka has 

 observed at least two layers of cells in the proximal or retinal section 

 and a differentiation of an inner layer of high cylindrical ependymal cells 

 and beneath these (more superficially) rounded cells. Among the former 

 Studnicka describes in Bufo, cells of a distinctly rod-like character, and 

 Galeotti has seen rounded protoplasmic masses projecting into the lumen, 

 like those in Petromyzon. The structure of the vesicle thus resembles 

 a degenerate sense organ rather than a gland, and its cells differ markedly 

 from those of the surrounding cutaneous glands. 



Nerves of the Parietal Organ and Epiphysis in Anura 



The fine strand of fibres which sometimes connects the " Stirndriise " 

 or " frontal gland " with the roof of the skull was first described by 

 Stieda in 1865. This slender band may contain nerve-fibres, or be 

 composed simply of connective tissue, or it may be absent. It is generally 

 considered to originate as the stalk of the pineal organ which in the 

 course of development has become thinned out and elongated, as repre- 

 sented in Fig. 161 (p. 228) by Braem, of the parietal region of a tadpole 

 of Rana temporaria. Nerve-fibres may appear in the stalk at a later 

 stage and it was thought at the time that the stalk was transformed into 



