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 fubuli 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 ureter of the 
same side, over the ventral surface of the kidney, and opens upon 
a small papilla into the urodeum 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 Passeres, 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 Ploceidx,! and has been observed in 
Accentor collaris. 
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 afford characters of no mean taxonomic value (cf. 
Ballowitz, Anat. Anzeiger, 1886, pp. 363-376, and Arch. mikrosk. 
Anat, xxxil. 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. 
1 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. 
