MUSEUM OF COMPARATIVE ZOOLOGY. 179 



At the stage represented in Fig. 1 2, the cavity of the pocket is scarcely 

 noticeable. It should appear, of course, between the second (r.) 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, mh.). 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 (PI. III. 

 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 Trom it. In the sagittal sec- 

 tion (PI. 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 



