OTHER RETINAL TAPETA 



239 



however (as in a couple of poorly developed ungulate tapeta) there may 

 be none at all in most of the cells. An occasional cell contains consider- 

 able pigment in the cell-body (European sturgeons) or a great deal of 

 pigment compactly massed in the tip of a very heavy process (Acipenser 



» 







■*i,it 



Fig. 95 — Retina of the common opossum, Didelphis virginiana. x415. After Walls. 



a, from the upper part of the tapetalized region (compare Fig. 93c), showing modification 

 of the pigment epithelium; a very few pigment granules are present, along with a mass of 

 reflective material, in this part of the tapetum (note capillary against external limiting mem- 

 brane, at right), b, from the inferior fundus, showing unmodified, heavily pigmented pig- 

 ment epithelium, contrasted with the tapetum in a (t) by the alignment of the external lim- 

 iting membrane in the two photos. 



fulvescens — Fig. 96). The effect is as though all the cells had pooled 

 their pigment in a scattered minority of their number, in order to min- 

 imize the obscuration of the tapetum. The elements of the tapetum 

 fibrosum in some marine teleosts (most of them bathypelagic) contain 

 large masses of guanin, which were formerly called 'ophthalmoliths'. 



