THE DEVELOPMENT OF LKPIDOSIEEN PARADOXA. 443 



The membrana limitaiis externa lies immediately external 

 to the visual cell-nuclei. Some of these nuclei do not reach 

 the membrane,, others are in contact with it and even bulge 

 it outwards. Occasionally one is also able to make out that 

 the membrane passes up round the principal vacuole (cf. 



fig. 11)- 



There is no differentiation of the fully formed retinal 



elements into rods and cones. The structure figured by 

 Schiefferdecker for Ceratodus ('Arch. mik. Anat./ Bd. 

 xxviii, Taf . xxiv^ fig. 80) is, I have no doubt, a young rod- 

 cell which has not yet fully developed its rod.^ 



Pigment Layer. — The pigment layer of the retina con- 

 sists of a single stratum of cells, polygonal, very usually 

 pentagonal, in outline as seen in surface view. 



The inner portion of the cell-body is laden with melanin 

 granules, and during the exposure to light this is extended 

 in a multitude of fine thread-like processes which pass up 

 between the rods and rod vacuoles, many of them reaching 

 to the membrana limitans externa. 



In a specimen of the same brood killed by faint lamplight 

 during the night the processes were somewhat shorter, more 

 clumped together, and more pigment was visible in the 

 general protoplasm of the cell-body. 



I shall now proceed to describe the features Avhich I have 

 been able to make out in the histogenesis of the retina, more 

 especially in regard to the development of the layer of visual 

 cells.^ 



Histological Evolution of the Optic Cup. 



Histological differentiation of the ojjtic cup becomes evident 

 about Stage ol. At this stage pigment granules are seen in 

 the posterior wall of the cup, appearing in each cell along the 



' Cl". tor frog, Bernard, this Juurnal, vol. icJ, page 30. 



- For the most modern conlribut.ion to tliis subject see Levi, "Osservazioni 

 sullo svilluppo dei coni e basloncini della lletine de<;li Urtideli," 'Lo Spcri- 

 meutale,' anno liv, 1900. 



