On the Pineal Eye of the Porbeagle Shark. 63 
similar to the primary optic vesicles of the lateral eyes. Two 
years later, Ahlborn ! also drew attention to this resemblance. 
But it was in 1886 that Graaf? maintained that the pineal 
eye.of Anguis fragilis resembles the highly-developed inver- 
tebrate eye, such as that of a Cephalopod, Heteropod, or 
Pteropod. This view of Graaf has been adopted more 
recently by Mr Spencer ® and Dr Beard.* 
Appearances presented by a longitudinal vertical section, made 
through the centre of the pineal body, its stalk, and the 
overlying skin. 
Almost directly over the expanded end of the pineal body 
(Plate II., Fig. 1), but separated from it by mesoblastic tissue, 
there is (1) a well-marked involution or depression (P.P.) 
of the epidermis or epiblast (Zp.), which at this spot is not 
only thickened, but is made up of at. least two distinct 
layers of cells (Fig. 2, Hp.), whereas the epidermis in the 
immediate neighbourhood of the depression is composed of 
only a single layer of cells. Below the epidermis lies (2) the 
thick stratum of clear embryonic mesoblastic connective 
tissue (Z.c.t.), in which are scattered a number of connective 
tissue-corpuscles, and in which the formation of cartilage (c.) 
has just begun at certain spots, but not immediately over the 
pineal body. In a shallow concavity or depression on the 
under surface of this tissue, and almost immediately under 
the epiblastic involution, lies the expanded distal extremity 
of the pineal body (P.B). From the ventral extremity of the 
latter stretches a long, slightly curved, and posteriorly-directed 
stalk (P.St.), which, like the body, is a hollow structure, and 
places the cavity of the body in direct continuity with the 
ventricles of the brain. The greater portion of the stalk, 
together with pineal body or “ vesicle,” is, however, embedded 
in (3) a somewhat thick stratum of very delicate protoplasm 
(c.c.t.), so transparent as to be recognised with difficulty under 
the microscope. In it lie scattered a few nucleated cor- 
puscles. The parts of interest are, however, the epiblastic 
1 Zeitschr. f. Wiss. Zoologie, 1884. 2 Zool, Anzeiger, March 29, 1886. 
3 Quart. Journ. Micros. Sci., 1886 (vol. xxvii). 
4 Quart. Journ. Micros. Sci., 1888-89 (vol. xxix.) ; Nature, July 14, 1887, 
