314 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



tudinal sections, and Figs. 4 and 5, which are cross-sections, the shape of 

 this cavity is shown. The membrane, which intervenes between the 

 optic chamber and the space in which the ganglion lies, is shown in all 

 four figures at z'. The rupture of this membrane in Fig. 2 is the point 

 of connection between these two blood spaces. The membrane is seen to 

 be continuous, on the one hand, with the delicate membrane around the 

 ganglion and, on the other hand, with the membrane upon which the 

 cells of the ectoderm rest. Figs. 1 and 2, Plate XLIX, show the charac- 

 teristic infolding of the ectoderm between the anterior part of the eye 

 and the dorsal surface of the ganglion. The only parts of the eye that 

 abut on the ganglion are the posterior ends of its two posterior limbs. 

 All the rest is separated from it by a double fold of ectoderm. In Fig. 5, 

 Plate XLIX, which is a vertical cross-section of the ganglion and eye, 

 through the region designated by e' in Fig. 1, there is shown the double 

 fold of ectoderm, fc, separating the eye, e', from the ganglion. The deli- 

 cate membrane that clothes the ganglion is continued over the whole 

 eye (Fig. 1). 



The arrangement of the histological elements is different in different 

 regions (see Plate XLIX, Figs. 1 and 2). In the posterior limbs, e', the 

 rod cells are dorsal and the pigment, p', ventral. In the anterior curved 

 part of the eye, e", this arrangement is nearly reversed ; the pigment, p", 

 being on the dorsal and posterior faces, while the rods are on the ventral 

 and anterior faces. Where these two regions, e' and e", meet at the 

 antero-lateral angles of the eye, the pigment layer of the one bends 

 toward, but is not quite continuous with that of the other; the two 

 almost meeting on the inner face of the eye, i. e., on that face which 

 looks toward the mid-dorsal point of the ganglion. There is, then, a 

 twisting of the main body of the eye, causing the pigment layer, as we 

 pass from behind forwards, to face first ventrally, then toward the 

 middle line, and finally dorsally and posteriorly. On the other hand, 

 the same twisting causes the rod cells to face first dorsally, then laterally, 

 and at last anteriorly and ventrally. 1 In the second, smaller portion of 

 the eye, which lies in the anterior curve of the main portion, the rod 

 cells face posteriorly toward the open end of the horseshoe, while the 

 pigment layer faces anteriorly, lying close to the pigment layer of the 

 anterior part of the main body of the dorsal eye. 



1 1 do not mean that there has been an actual twisting, but that there is seen in 

 the eye a spiral arrangement of its elements such as would result from twisting. The 

 development (see below) shows there has been no actual twisting. 



