THE PINEAL BODY 93 



Frequently the wall of the vesicle presents reduplication, as in 

 the case of Spinax niger where there is a distinct tendency to 

 tabulation, or as in Acanthias where the folding of the wall 

 results in the production of two adjacent vesicles. In a single 

 instance only is there a marked differentiation between the 

 ventral and dorsal walls. This occurs in Lamna cornubica, 

 particularly in the embryonic state, described by Carrington 58 

 in 1890. In this form the under wall was thicker than the 

 dorsal wall. Studnicka 389 found some tendency to such a dif- 

 ferentiation in Spinax. 



Hist ologic ally, the walls of the end- vesicle are made of epen- 

 dymal cells, but there are no cylindrical or spindle cells to be 

 observed in this structure. The cells described in cyclostomes 

 as having prolongations of such a character as to warrant the 

 description of ciliated cells are absent in selachians so that no 

 such processes make their way into the lumen of the end-vesicle, 

 as is the case in Petromyzon. The nuclei of these cells are 

 situated at varying distances from the surface of the wall so that 

 the ependyma gives the impression of stratified epithelium, 

 whereas in reality it is a single layered epithelial structure. 

 Some cells have a rather long process which approach, but do 

 not enter, the lumen of the end-vesicle. This manifestation is 

 taken as a probable sign of an excretory function of the cells in 

 question. Galeotti 140 in 1896 described in Scyllium peculiar 

 appearances which seemed to indicate a secretory or excretory 

 activity on the part the cells in this portion of the pineal 

 organ. Among the more usual cells, according to Studnicka, 389 

 there are many smaller cells scattered here and there of a similar 

 type to the sense cells in the retina of Petromyzon. The signifi- 

 cance of these cells is not at all clear, and Studnicka himself is 

 not willing to accredit them with a definitely receptor function. 



The stalk of the pineal organ. Microscopically, this appears to 

 be a long, narrow strand connecting the end-vesicle with the 

 roof-plate of the interbrain. Upon microscopic examination 

 it is found, however, to contain a central but narrow lumen, the 

 entire structure, therefore, being tubular. In most instances 

 this stalk maintains an equal diameter throughout its entire 



