THE DIPNOAN RETINA 



591 



r^ 



The processes are numerous, long, and filamentous (Fig. 20d, p. 44). 



All of the retinal elements are monstrous, as are the cells in most of 

 the organs of lepidosirenids. In Protopterus the outer nuclear layer con- 

 tains two rows, each incomplete — more nuclei lie above the excessively 

 delicate limitans than below it, and both rod and cone nuclei may occur 

 in either location. The inner nuclear layer consists of four compact rows; 

 and if horizontal cells are present, their cytosomes are as slenderly fibrous 

 as those of the highest vertebrates. The outer plexiform layer is extreme- 

 ly thin, the inner plexiform thick as usual. There is a single 

 row of ganglion cells. The optic nerve of Protopterus is a 

 slender and simple cord, with an ependymal core as in lam- 

 preys; but in Lepidosiren and Neoceratodus the nerve fibers 

 are blocked off by glial septa into fascicles, each with an 

 axial core of (ependymal?) nuclei. 



Protopterus has the most elab- 

 orate visual-cell pattern (Fig. 171 

 and Plate I). The rod exhibits a 

 maximum of cone-like morpholog- 

 ical features: it not only has the 

 same cone-like (i.e., particulate) 

 nuclear chromatin as the rods of 

 most lower vertebrates and the 

 cones of all, but it also has a huge 

 oil-droplet and a paraboloid (cf. 

 Figs. 22, 23; pp. 54-5). This rod 

 has certainly been secondarily de- 

 rived from a cone, and the chances 

 are that it is archaic, and represents 

 the primitive chondrostean rod, 

 changed but little or not at all. 

 On the pathway leading toward the teleosts this rod promptly lost its 

 oil-droplet (as did the cones at the holostean level, where the light-loving 

 Amia has had to replace them with a yellow cornea) ; but here in the 

 lungfishes the oil-droplet has persisted. There remains of course a possi- 

 bility that the lungfish rod has been derived from a lungfish single cone. 



In Lepidosiren, according to Kerr, there are only elements which seem 

 identical with the rods of Protopterus. Neoceratodus, according to the 

 half-century-old observations of Schiefferdecker, has only single cones 

 with oil-droplets and rods without them. 



Fig. 171 — Representative visual cells of 

 African lungfish, Protopterus athiopicus: 

 single cone, double cone, and rod. x 1000. 



