64 Proceedings of the Royal Physical Society. 
involution or pit, the dilated portion of the pineal body or 
primary optic vesicle, and the hollow stalk of the latter. 
1. The Epiblastie Involution or Pit—on surface view before 
the sections were made—appeared as a distinct though small 
circular depression quite visible to the naked eye, in the 
mid-dorsal surface of the head of the embryo shark. On 
examining a longitudinal vertical section of the pit, a secondary 
depression (s.p., Fig. 1), even with comparatively low powers 
of the microscope, was distinctly observable. As this second- 
ary depression was noticed in a large number of sections, 
it was probably natural, and not due to shrinkage or any 
other untoward accident. The epiblast was thickened through- 
out the whole extent of both the primary and secondary 
depressions, and consisted of at least two layers of cells. Most. 
of the cells resembled those of the epiblast in the rest of the 
head-region, 7.¢., they were circular or ovoid in section, each 
with a comparatively large nucleus, whilst here and there 
occurred a few large cells (m.c., Fig. 2), usually in groups, with 
clear contents and small nuclei (probably mucous cells). A few 
such cells, however, occurred in the adjoining a but 
they were not, as a rule, in groups. 
2. The Primary Optic Vesicle or Pineal Body.—In sections 
passing through the centre of the vesicle, its outer surface is 
seen to be distinctly conical, its upper or dorsal surface con- 
vex, and occupying a depression in the embryonic connective 
tissue—almost immediately under the epiblastic involution. 
The central cavity of the pineal body, though conforming in 
a general way to the outer conical surface of the vesicle, is 
sufficiently irregular to call for remark. On looking at the 
figure, it is at once seen that the anterior wall of the vesicle 
(i.e, the side to which the reference letter P.B. points) is 
very much thinner than the posterior wall; further, that 
the cells forming the wall of the vesicle are of very unequal 
lengths, certain of the cells projecting so far into the cavity 
of the vesicle as to appear as small hillocks—three such 
hillocks are seen in the figure. The cells—especially in the 
thicker parts of the wall of the vesicle—are elongated 
columnar cells, each with two, three, or more nuclei—the 
nuclei lying in the long axis of each cell, and occupying that 
