684 MAMMALS 



is very nearly so in many murid rodents and in a few other small-eyed, 

 nocturnal, lower placentals. In carnivores and ungulates it is variously 

 intermediate in shape; and its relative size is always related to the habits 

 of the animal with respea to light (Fig. 71, p. 173). 



The Retina — The pigment epithelium usually contains relatively little 

 pigment, which is never migratory (Fig. 20a-c, p. 44). It may contain 

 reflective material and serve as a tapetum lucidum in itself (opossum, 

 Megachiroptera) , or in aid of a chorioidal tapetum (dog). Not all 

 fruit-bats have the reflective substance — it is lacking in most species of 

 Pteropus, but is abundant in Pteropus h. condorensis, Hypsignathus, 

 Cynopterus and Epomophorus. It is not apparent whether these differ- 

 ences relate to differences in the strictness of noctumality of the various 

 genera. 



Usually the placentalian retina is described as being, typically, vascu- 

 larized. Actually, retinal vessels, with capillary branches passing out 

 ordinarily as far as the outer plexiform layer, are numerous only in 

 primates, sciurids, carnivores, and artiodactyls — all, characteristically di- 

 urnal or arhythmic (see pp. 654-5). In the primitive ruminant Tragulus 

 (the mouse-deer or chevrotain), there are only superficial vessels, like the 

 hyaloid or vitreal vessels of ichthyopsidans and snakes. In most perisso- 

 dactyls there are no vessels, and in the horse they are restricted to a 

 six-millimeter circle concentric with the disc. There are but few vessels 

 in lagomorphs, associated there with the horizontal band of medullated 

 nerve fibers; and there are few or none in the various rodents outside 

 the Sciuridse. There are no vessels in the Xenarthra, or in the Chiroptera 

 except for a few superficial capillaries in Pipistrellus pipistrellus. Retinal 

 vessels thus seem to have arisen several times, independently, in those 

 placental mammals with the most cones in their retinae; and certain 

 embryological differences appear to bear out this conclusion. In murids, 

 for example, the few adult retinal vessels are formed directly by the 

 embryonic vasa hyaloidea propria, whereas in primates these atrophy, 

 and the definitive vessels bud out from the central retinal artery and 

 vein in the optic-nerve head. 



The lamination and the laminal purity of the placentalian retina are 

 only ordinary, and quite well exemplified by the human retina (Fig. 19, 

 p. 43 ) . Only in the diurnal squirrels and particularly in the prairie-dogs 

 (Cynomys spp.), and there only in the dorsal region, does the mam- 

 malian retina approach that of the birds in the segregation of inner- 



