10 ARTHUR DENDY. 



extremities, but the knobs of the latter project further into 

 the cavity of the eye than those of tlie former. The knobs of 

 the pigment cells are much broader than the sense-cells at 

 the same level, so that they form almost a continuous layer 

 inside the limiting membrane, penetrated by the slender rods 

 of the sense cells on their way to the sense cell knobs (fig. 5, 

 S.G.K.) in the cavity of the optic vesicle. 



The nuclei of the pigment cells (fig. 5, N.P.C.) are situated 

 towards the outer extremities of the outer segments, at about 

 the same level as the nuclei of the sensory cells, from which 

 they may be distinguished by their somewhat smaller size 

 and less dense-looking protoplasm. The pigment granules 

 (composed of phosphate of lime ?) are minute spherical bodies 

 evenly distributed throughout the inner segment and the 

 greater part of the outer segment, but not, so far as my 

 observations show, occurring in the slender outermost portion 

 of the pigment cell beyond the nucleus. 



Examination of Studnicka's earlier figures (7, PI. Ill, figs. 

 Q, 7, 8) suggests that in Petromyzou also the pigment cells 

 may have differentiated knobbed inner extremities. The idea 

 that only one kind of cell is present in the retinal epithelium, 

 as shown in these figures, is doubtless, as Studnicka himself has 

 since pointed out, erroneous. According to his earlier obser- 

 vations, however, all the epithelial cells have knobbed (but 

 unpigmented) extremities, and it appears just possible that 

 he has abandoned too much of his previous results in making 

 the necessary correction. In spite of the precision of his 

 later account, it might be worth while to re-investigate this 

 point in the light of our knowledge of Geotria, in which 

 the segmentation of the pigment cells is so obvious as to 

 leave no room for doubt. 



The nervous layer of the retina consists of ganglion cells, 

 nerve fibres, and connective-tissue cells. The ganglion cells 

 (figs. 5, 7, G.C.) are very conspicuous on account of their 

 large spherical nuclei, surrounded by only a small quantity 

 of cytoplasm. The cytoplasm is often scarcely recognisable, 

 while at other times it is more distinct and exhibits a multi- 



