98 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 
Miller 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 Ammoccetes, but must, therefore, be searched for in the 
opposite direction. 
Miller 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 Reichenbach, 
