MUSEUM OF COMPARATIVE ZOOLOGY. 179 
At the stage represented in Fig. 12, the cavity of the pocket is scarcely 
noticeable. It should appear, of course, between the second (7.) and 
third (p r.) layers, and at the deep end of the infolding a trace of it is. 
visible (cav.). The second and third layers, however, are quite distinct, 
and show no indications of fusion. The cavity of the pocket is oblit- 
erated only by its opposite walls coming in contact, so that even in 
Fig. 12 a pocket may be spoken of without inconsistency. In stages 
earlier than that given in Fig. 12, the cavity of the pocket is very no- 
ticeable, and from its external opening to its deep end it is a continuous 
open space. 
In a horizontal section of the earliest stage examined, the region just 
above the external opening of the pocket presents the appearance of a 
slightly irregular tube cut crosswise (Pl. III. fig. 13). The wall of the 
tube is made up of a single layer of hypodermis, whose deep surface 
is covered with a delicate basement membrane (fig. 13, mb.). The cavity 
of the tube is continuous with the pocket of the infolding (fig. 13, cav.). 
At about half the distance from its opening to its deep end, the pocket is 
divided in the median plane into a right and left compartment (Pl. II. 
figs. 14, 15). Each compartment has the form of a sac flattened from 
before backwards. The sacs extend dorsally on either side of the median 
plane, and end blindly. 
One can distinguish, then, in the invagination a common neck, and 
two symmetrically placed sacs which arise from it. In the sagittal sec- 
tion (Pl. III. fig. 12) already described, the thin ventral third of the 
infolded hypodermis corresponds to the neck, and the thickened dorsal 
two thirds to the anterior wall of the sac. The position of the sacs is 
indicated externally by the areas of pigment already alluded to; the sacs 
are destined to become the retinas. The neck soon disappears, but some 
time before this takes place the outer wall of each sac is thickened still 
more and becomes more deeply pigmented. The thickened faces form 
the essential part of the retina, with which, after the closure of the pocket, 
the posterior thinner layer fuses. 
The three hypodermal layers which enter into the composition of the 
eye, have received special names. That portion of the permanent hypo- 
dermis which is directly external to the optic sac, constitutes the first 
layer. At a later stage it produces the lens, and consequently has been 
termed by Mark (’87, p. 77) the “lentigen.” By other authors it has 
been generally designated as the “vitreous.” Directly under the lentigen, 
and forming the thick external wall of the optic sac, is the second or 
retinal layer. Behind this layer the thin internal wall of the sac forms 
