310 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



ectoderm. On the mid-line in front of the ganglion there is a perfora- 

 tion of this membrane, through which the circumganglionic blood space 

 freely communicates with the optic chamber. The delicate membrane 

 that clothes the ganglion is continued over the optic ridge. The mem- 

 brane that closes off the optic chamber from the space in which the 

 brain lies is continuous, on the one* hand, with the membrane that 

 covers the ganglion and eye, and, on the other hand, with the base- 

 ment membrane upon which the ectoderm cells abut. That portion of 

 the ectoderm which covers the eye and bounds the optic chamber I pro- 

 pose to call the optic sheath (Plate LI, Fig. 8, os). 



The histological structure of the retina is the same in all regions. 

 Fig. 8, Plate LI shows a cross-section of the ganglion and of the two 

 anterior limbs of the optic ridge. Figure 9 is a more enlarged drawing 

 of the retinal portion of the left limb shown in Fig. 8. The retina con- 

 sists of three kinds of cells: (1), rod cells; (2), intermediate cells ; (3), pig- 

 ment cells. The rod cells (Fig. 9, r and s) are elongated columnar ; their 

 long axes being parallel to the horizontal plane of the body of the salpa. 

 They each consist of two portions: s, a thin-walled, lightly staining, 

 outer portion (away from the core of the ridge) containing very finely 

 granular protoplasm and a large nucleus with nucleolus and very 

 apparent chromatin network; and r, a thick- walled, deeply staining, 

 inner portion abutting on the next inner layer (2). The deep stain of the 

 inner third of the rod cells is due to the thickness of the cell walls, 

 which take the stain much more readily than the protoplasm does. 



In the second layer (2), (Fig. 9, Plate LI, i) no cell boundaries can be 

 made out, but the presence of a number of nuclei exactly resembling the 

 nuclei of the peripheral portion of the ganglion indicates the cells of the 

 second layer, for which I propose the name intermediate cells of the retina. 

 The cell boundaries in the peripheral portion of the ganglion are no more 

 visible than in the intermediate layer of the retina. These intermediate 

 cells are ganglion cells which have remained unaltered during the 

 development of the eye from the cells of the brain. In each section the 

 number of intermediate cells is about equal to the number of rod cells. 

 I have been unable to study live salpas, so could not make successful 

 macerations. I am convinced, however, that the intermediate layer of 

 the retina is not a multinucleated mass of protoplasm, but is composed 

 of true cells. In a partially successful maceration of the peripheral 

 layer of the hardened ganglion, whose cells exactly resemble in appear, 

 ance the intermediate cells of the retina, I could make out that the gang- 

 lion cells were of irregular shape and had, in some cases, as many 



