9§ THE ORIGIN OF VERTEBRATES 



optic lobes and other parts of the Ammoccetes brain, or in the cortical 

 layers of the optic ganglia of all arthropods. 



Between this small-celled nuclear layer (4, Fig. 41) and the layer 

 of nuclei of the visual rod cells (7, Fig. 41) (the external nuclear 

 layer), we find in the eye of Ammoccetes and Petromyzon two well- 

 marked rows of cells of a most striking character — viz. the two 

 remarkably regular rows of large epithelial-like cells with large 

 conspicuous nuclei, which give the appearance of two opposing rows 

 of limiting epithelium (5, Fig. 41), already mentioned in connection 

 with the researches of Langerhans and W. Miiller. Here, then, is a 

 striking peculiarity of the retina of the lamprey, and according to 

 Miiller the obliteration of these two layers can be traced as we pass 

 upwards in the vertebrate kingdom. Among fishes, they are especially 

 well seen in the perch ; in the higher vertebrates the whole layer is 

 only a rudiment represented, he thinks, by the simple layer of round 

 cells which lies close against the inner surface of the layer of 

 terminal fibres (Nervenansatze), and is especially evident in birds 

 and reptiles. In man and the higher mammals they are probably 

 represented by the horizontal cells of the outer part of the inner 

 nuclear layer. 



Seeing, then, that they are most evident in Ammoccetes, and 

 become less and less marked in the higher vertebrates, it is clear 

 that their origin cannot be sought among the animals higher in the 

 scale than Ammocoetes, but must, therefore, be searched for in the 

 opposite direction. 



Miiller describes them as forming a very conspicuous landmark in 

 the embryology of the retina, dividing it distinctly into two parts, an 

 outer thinner, and an inner somewhat thicker part, the zone formed 

 by them standing out conspicuously on account of the size and regu- 

 larity of the cells and their lighter appearance when stained. Thus 

 in his description of the retina of an Ammoccetes 95 mm. in length, 

 he says, " The layer of pale tangentially elongated cells formed a 

 double layer and produced the appearance of a pale, very charac- 

 teristic zone between the outer and inner parts of the retina." 



Let us now turn to the retina of the crustacean and see whether 

 there is any evidence there that the retina is divisible into an outer 

 and inner part, separated by a zone of characteristically pale staining 

 cells with conspicuous nuclei. The most elaborate description of 

 the development of the retina of Astacus is given by Eeichenbach, 



