On the Pineal Eye of the Porbeagle Shark. 65 
portion of the cell lying nearest the central cavity. The 
cavity of the vesicle contained some protoplasmic (cerebro- 
spinal) fluid, which had been coagulated by the alcohol used 
for preserving the specimen. The presence of cilia could not 
positively be asserted, but the appearance of some of the 
cells seemed to indicate that the central cavity of the vesicle 
had in all probability been ciliated. 
3. Stalk or Stem of the Pineal Body.—Tubular, slightly 
curved, with its proximal or central end directed slightly 
backwards. The epithelium lining the cavity of the stem 
and forming its wall is composed of columnar cells of 
nearly uniform length and thickness—the nuclei lie nearest 
the central ends of the cells, which appear in certain sections 
as if they were ciliated. The lumen of the stalk is in direct 
continuity with the cavity of the pineal body, and, like the 
latter, contains a little coagulated cerebro-spinal fluid. 
4. Connective-Tissue Sheath—A fibrous sheath (et.s.) 
encloses the pineal body and its stalk. Owing probably to 
shrinkage, a distinct space is left between the sheath and 
the outer surface of the pineal body (P.B.). 
I should have stated, when describing the pineal vesicle, that 
the sections some distance away from the median longitudinal 
vertical plane appear to indicate that the lateral wall of 
the vesicle is plicated or pushed in at intervals, and that 
this infolding is not due to any shrinkage produced by 
alcohol. 
The question naturally arises, What is the morphological 
signification of the thickened involuted portion of the super- 
ficial epidermis? Is it the first beginning of the future lens 
of the pineal eye, exactly comparable to the first step in the 
lens formation of each of the lateral eyes, arrested in fact at 
the stage which has been described by Balfour,! thus :—“The 
epiblast in front of the pit becomes very much thickened 
and then involuted as a shallow pit.” If this median involu- 
tion of the epiblast in LZ. cornubica is of any significance at 
all, or rather has the significance that the involution of the 
-epiblast in connection with the lens formation of each of 
the lateral eyes has, then it follows, as a matter of course, 
1 Development of Elasmobranch Fishes, p. 184. 
VOL. XI, E 
