MUSEUM OF COMPARATIVE ZOOLOGY. 189 
middle area. This granular substance (PI. IT. ‘fig. 4) extends down 
between the rhabdomes, and merges with a less regularly granular sub- 
stance behind. The rhabdomes at their deep ends merge imperceptibly 
into this, irregularly granular substance. In the region where the deep 
ends of the rhabdomes disappear, large slightly granular nuclei occur 
(Fig. 4, nl.7.). All the nuclei in the retina of Centrurus are found in 
what has been described in the pigmented eye as the mottled area. ‘The 
nuclei nearest the concave surface of the retina are the largest, and, as 
has been previously mentioned, are slightly granular. Behind these, in 
the middle of the nuclear region, smaller oval nuclei (Fig. 4, nl. pig.) 
occur. Here also nerve fibres are abundant, and those curious bodies 
mistaken by Graber for nuclei and designated by Lankester and Bourne 
under the name of phaospheres (Fig. 4,-pha sp.). The deepest nuclei 
(Fig. 4, nd. pr.) in the retina are flattened, and more deeply colored than 
the rest. They lie upon the internal surface of the densely pigmented 
zone, previously mentioned, and form a line of separation between that 
zone and the coarsely granular substance in front. The substance of the 
deepest zone is almost identical in character with that of the outer portion 
of the retina, and its granular appearance, like that of the external layer, 
is largely due to the colorless remains of pigment granules. The smaller 
anterior and median nuclei of Graber do not exist in Centrurus, either 
in the retinal cells or between them, as claimed by Lankester and Bourne 
(83, pp. 192, 193). The latter authors state (83, p. 192) that the 
reason Grenacher overlooked these nuclei was that the acid which he 
used to remove the pigment destroyed them. In the case of Centrurus 
sections depigmented with the 35% mixture of nitric and hydrochloric 
acids, with }% solution of potassic hydrate, or unaffected by depigmenting 
reagents, but colored with borax-carmine and cut three micromillimeters 
thick, show no trace whatever of anterior or middle nuclei. The exami- 
nation of fresh material and of maceration preparations has given the 
same results. Moreover, since the nuclei in the brain after treatment 
with 1% potassic hydrate are not to be distinguished from those in the 
same organ unaffected by that reagent, it seems scarcely possible that the 
same reagent could destroy nuclei in the retina. It is therefore safe to 
conclude that at least in the retina of Centrurus no nuclei exist external 
to the band of larger nuclei already described. 
Having shown that the retinal nuclei are limited to the deeper region 
of the retina, and that these nuclei are of three principal types, we are 
now prepared to inquire into the cellular composition of the retina. 
This is best done by means of isolation preparations. In the retina 
