8 ARTHUR DENDT. 



number of almost spherical nuclei of doubtful significance, 

 but resembling the nuclei of the ganglion cells of the retina. 

 At various levels in the pellucida one also finds a small 

 number of very darkl}- staining, small, irregular nuclei, having 

 a shrivelled appearance, and closely resembling the " con- 

 nective tissue " nuclei found in the interior of the optic 

 vesicle and in the nervous layer of the retina. 



Histology of the ^jetuLa-r — The retina, as already 

 indicated, may be divided into two perfectly distinct layers, 

 the epithelial layer, composed of pigment cells and sense cells, 

 and the layer of ganglion cells and nerve fibres which lies 

 behind it, and which we may call, in short, the nervous layer. 

 Both these layers increase greatly in thickness as they recede 

 from the pellucida, and around the atrium the nervous layer 

 becomes so strongly developed as to form a veritable ganglion 

 (fig- 7). 



The epithelial layer of the retina (figs. 5 and 7) is composed 

 of the same two kinds of elements as have been recognised by 

 Studnicka in Petromyzon — viz. sensory cells and pigment 

 cells. The former (fig. 5, B.S.C.) are greatly elongated, 

 slender rods whose inner ends project into the cavity of the 

 optic vesicle and terminate in irregularly rounded, swollen 

 knobs, while their outer ends branch into fibrils which lose 

 themselves in the fibrillar network of the nervous layer. These 

 rods have large oval nuclei (fig. 5, N.8.C.) situated towards 

 their inner ends, and causing a fusiform swelling in the rod 

 itself. The end-knobs of the rod (fig. 5, S.C.K.), and the rods 

 themselves (apart from the nucleus), are only lightly stained 

 with Ehrlich's hfematoxylin, but take up acid fuchsin with 

 great avidity, whereby they are rendered very conspicuous. 

 Studnicka describes the end-knobs in Petromyzon as being 

 differentiated into inner and outer portions, but I have not 

 succeeded in detecting any such differentiation in the case of 

 Geotria, the knobs appearing to be practically homogeneous. 

 The adhering ends of the protoplasmic strands (fig. 5, P.St.) 

 which connect the sense-cells with the pellucida, may, however, 

 spread out on the knobs, and thus give rise to the appearance 



