esterly: eucalanus. 15 



the investigations of Parker ('91, p. 81) have shown that there are two 

 cone cells in each retina, in addition to eight retinular cells and "other 

 nuclei which probably represent undifferentiated cells." 



It is plain that the number of retinal elements is accurately known 

 only in Eucalanus elongatus and E. attenuatus, and that in these 

 species the number is definite and constant both as regards the paired 

 and unpaired portion of the eye. 



The nuclei of the retinal cells are spheroidal or ovoidal and contain 

 a chromatin network. The nuclei of other cells which appear around 

 the eye (such as muscle cells, the hypodermis cells, and sheath cells) 

 may be distinguished at a glance from those of the retinal cells by 

 the difference in staining reaction. This of course appears to best 

 advantage in haematoxylin stains, but is also evident in Mallory's 

 connective-tissue stain. Figure 16 (Plate 2) shows the differences 

 accurately, and Figure 23 (Plate 2) more diagrammatically. It will be 

 seen that the nuclei of the retinal cells invariably contain more chro- 

 matin and so stain more deeply. This is an important character, for 

 otherwise the nuclei of muscle or connective-tissue cells would be con- 

 fused with them. It may be noted, also, that aside from their location, 

 the nuclei of the optic cells are indistinguishable from those of the cells 

 in the brain. 



e. Interior bodies (Phaosomes); their Arrangement in the Cells. — 

 The nuclei, however, are not the most noticeable bodies within the 

 retinal cells. This characteristic belongs to the "interior bodies" 

 (Binnenkorper), which Hesse (:01, p. 350) was the first to describe. 

 He found that the interior bodies lie between the nucleus and the 

 particular structure (Stiftchensaum) which, in his opinion represents 

 the ending of the retinal nerves, but nearer the nucleus. These bodies 

 have a greater affinity for stains, as Hesse has stated, than does the 

 cell plasma. Hesse considers that they are essentially ribbon-like 

 bodies, which may be so sharply bent or twisted that they seem to be 

 divided into separate pieces. He thinks that branching of the bodies, 

 if it occurs at all, is rare. He failed to find these bodies in Eucalanus 

 attenuatus, but they are, he states, of constant occurrence in Calanus 

 gracilis. He is imwilling to commit himself as to the part the interior 

 bodies play in the reception of light, but states that they are not neces- 

 sary for that function because they are absent from the retinal cells 

 of Eucalanus attenuatus. 



The observations of Hesse upon the median eye were made pri- 

 marily to determine the method of nerve termination in the retinal 

 •cells, and he has given scarcely any attention to the interior bodies. 



