670 



MAMMALS 



There are no blood vessels either in the retinal tissue (as in a few 

 marsupials and many placentals) or lying on its surface like the hyaloid 

 or vitreal systems of lower vertebrates. No monotreme has any trace of 

 a conus papillaris. This complete nutritional dependence of the retina 

 upon the chorioid is characteristic of light-shunning vertebrates (see pp. 

 648-58). The disc is small, smooth, and unpigmented in both genera, 

 circular in Ornithorhynchus and vertically oval in Tachyglossus. Kolmer 

 describes a peculiar mass of connective tissue which is embedded in the 

 bulbar portion of the optic nerve in Zaglossus. 



Fig. 195 — The visual cells of the lower mammals. xlOOO. 



a, single cone, double cone, and rod of Ornithorhynchus. After O'Day. 



b, element from pure-rod retina of Tachyglossus. Drawn from a preparation of O'Day. 



c, droplet-bearing and droplet-free single cones, double cone, and rod of an opossum, Mar- 

 mosa mexicana (Australian marsupials have no droplet-free cones and have droplets in both 

 members of their double cones). 



Not only in its avascularity, but in its entire histology, the mono- 

 treme retina is sauropsidan and might easily be taken for that of a 

 nocturnal reptile. In Ornithorhynchus, O'Day figures three rows of 

 outer nuclei, four of inner, and a single row of ganglion cells, and says 

 that the nerve-fiber layer is thin even near the disc. Tachyglossus, which 

 is pure-rod, has three layers of outer nuclei (Zaglossus has four) , only 

 two of inner (Zaglossus has three) , and a decidedly scattered single row 

 of ganglion cells. Some of the latter are ectopic and lie at various levels 

 in the inner plexiform layer. The greater extent of summation in the 



