784 RETINA 



line wide, but at the time of laying eggs it becomes more than two 

 feet in length and nearly half an inch in width, thus increasing its 

 volume about fifty times ; and this remarkable change takes place 

 annually. 



II. In the Male, the Testes are a pair of whitish-yellow glands, 

 of oval or globular shape — occasionally (as in Cypselus) vermiform — • 

 and lie at the anterior end of the Kidneys, being kept in position 

 by an enveloping peritoneal lamella, whence septa extend into the 

 interior. Within the meshwork thus formed are embedded the 

 spermatic vesicles or tuhuli seminiferi, which combine toward the 

 median side of each testis into wider tubes that in their turn leave 

 it, and joining numerous convoluted canals, the whole constitute 

 the Epididymis, which is irregular in shape and as a rule of a 

 deeper colour. Generally the left testis is bigger than the right, 

 although both are equally functional. During the breeding-season 

 they are greatly enlarged, as has been most often remarked in the 

 case of the House-Sparrow, where they increase from the size of a 

 mustard-seed to that of a small cherry, temporarily displacing the 

 usual arrangement of intestine, liver and stomach. The canals of 

 each epididymis unite to form a narrow tube, the vas deferens, that, 

 with small undulations, passes laterally along the lu-eter of the 

 same side, over the ventral surface of the kidney, and opens upon 

 a small papilla into the urodseum of the CLOACA (p. 90). The 

 walls of the vasa deferentia are furnished with unstriped muscular 

 fibre, but are devoid of glands, and there are no accessory glands, 

 seminal or prostate. In many birds, especially the Fasseres, the 

 vasa deferentia increase considerably in length during the breeding- 

 season, and form a closely convoluted mass which often causes 

 a protrusion of the cloacal walls, a peculiarity that is particularly 

 remarkable in some of the Ploceidse,^ and has been observed in 

 Accentor collar is. 



The spermatozoa of Birds,, though extremely minute, have a 

 complicated structure, the different parts of which present so many 

 differences of shape, size and proportion in various groups, that 

 they may possibly aflbrd characters of no mean taxonomic value (c/. 

 Ballowitz, Anat. Anzeiger, 1886, pp. 363-376, and Arch, mihrosk. 

 Anal xxxii. pp. 402-473, tabb. 14-18). 



RETINA, the visual or perceptive screen formed by the 

 terminal expansion of the optic nerve and lining the inner 

 chamber of the Eye. 



^ The external protrusion thus caused in certain of the South -African 

 Weaver-birds is often visible in their prepared skins, for it dries into a hard 

 hook-shaped excrescence and has given rise to various absurd and speculative 

 explanations. 



