MUSEUM OF COMPARATIVE ZOOLOGY. 189 



middle area. This granular substance (PL II. -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, 111. r.). 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, 111. p r.) 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 witli 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 J% 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 Centrums 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 



