THE PIGMENT-CELLS OF THE PINEAL BODY 65 



to contain white pigment, which Parker believed had the function of 

 reflecting rays of light back to the central part of the rhabdome, when this 

 was exposed by retraction of the black pigment away from the central 

 segment of the rhabdome (Fig. 44). It may be presumed that this action 



vimM%r- cl.rtn' 



• •Ware?* 1 1 



tmM&F rhb.- 



°°z°$°&- ac.c- 



HSPSi-- cl.rtn! 



B D 



Fig. 44. — Gammarus Ornatus (Freshwater Shrimp). Photochemical Changes 

 in the Retinal Pigment of Gammarus. (Aeter G. H. Parker.) 



Changes due to exposure to light are limited to the black pigment in the middle 

 and proximal portions of the retinular cells, they are not observable in the 

 accessory pigment cells (white pigment). 



A — Specimen fixed after exposure to light. The pigment has left the body of the 

 cell and has accumulated round the rhabdome. 



B — Transverse section of retinula through region of rhabdome. 



C — Longitudinal section through the retinula when light has been excluded. 



D — Transverse section through the retinula when light has been excluded. 



ac. c. : accessory retinal cell. 



b.m. : basement membrane. 



con. : crystalline cone. 



cl. cm. : corneal cell. 



cl. rt.n. : central retinal nerve cell. 



cm. : cornea. 



n. ac. c. : nucleus of accessory cell. 

 n. rt. n. : nucleus of retinal nerve- 

 cell. 

 rhb. : rhabdome. 



-> <- Arrows indicating the level of the transverse sections B and D. 



of the white pigment would come into play when the eye was exposed to 

 a dim light, and compensate in a measure for the loss of a bright illumina- 

 tion. Whether it is of the same nature as the white pigment of the 

 " pineal eye " of Petromyzon has not been demonstrated, but the presence 

 5 



