ON THE ORIGIN OF VERTEBRATES. 243 



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 mammalia they are probably repre- 

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

 nuclear layer. 



Seeing, tben, that they are most evident in Ammocoetes, 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 land- 

 mark 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 regularity of the cells and their lighter 

 staining appearance. Thus in his description of the retina of an 

 Ammocretes 95 mm. in length, he says : ^ " The layer of pale 

 tangentially elongated cells almost throughout formed a double 

 layer and produced the appearance of a pale, very characteristic 

 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 character- 

 istically pale staining cells with conspicuous nuclei. The most 

 elaborate description of the development of the retina of Astacus 

 is given by lieichenbach,"- according to whom the earliest sign 

 of the formation of the retina is an ectodermic involution 

 (Augen-einstiilpung), which soon closes, so that the retinal area 

 appears as a thickening. In close contiguity to this thickening, 

 the thickening of the optic ganglion arises, so that that part of 

 the optic ganglion which will form the retinal ganglion fuses 

 with the thickened optic plate and forms a single mass of tissue. 



^ Op. Cit. , p. XXV. 



' Entwick. Geschichtc d. FlusgJcrchses. Ahliandl. d. Scnckcnbcrgischcn Naturf, 

 Gcsell., Bdxiv., 1886. 



