FISH FOR EXPERIMENTATION 



361 



blackish-brown while the cultivated species is oi-ange or white, with or without black 

 spots. There are no color changes with breeding activity. The embryos begin to show 

 coloration about one week after hatching. (See references to Briggs, Egami). 



The sexes n:iay be distinguished by the following characters: 



MALE 



FEMALE 



REPRODUCTIVE PHYSIOLOGY 



The ovary of Oryzias is single and enormous, making the female easily distinguishable 

 from the male. In the mature fish it contains hundreds of follicles, each with an egg in 

 some maturation stage. The ovary itself is saucer-shaped with a membrane across its 

 dorsal surface, the various sized follicles projecting into the lumen beneath this mem- 

 brane. 



The mature eggs rupture their follicular walls, without hemorrhage, and emerge into the 

 ovarian lumen. This process requires from 20-60 minutes (see Robinson and Rugh, 

 1941) and generally takes place shortly before daylight. There is slight constriction of 

 the egg as it emerges, with evidence of muscular aid from the follicular wall or the 

 ovarian stroma. 



The maturational processes are apparently at their peak during the quiescent period of 

 sleeping and coincidental with revived activity at dawn, the eggs are ruptured and ovi- 

 position occurs. There is no doubt that the light cycle in some way regulates the egg- 

 laying periods, although other factors such as food, pH, and temperature cannot be ig- 

 nored. 



The eggs are held in the ovarian lumen until all the matured ones have ovulated and ovi- 

 position of all of the eggs occurs during a very brief period. A single female may pro- 

 duce from 1-80 eggs per day, the average being between 20-30. During a single season 

 500-800 eggs may issue from a single female, more being laid on sunny than on rainy 

 days. 



The oviduct is a single, muscular, and non-glandular tube about 1 mm. in length extend- 

 ing from the posterior ventral margin of the ovary to the exterior just posterior to the 

 urino-genital papilla. A ventral aspect of the female shows this opening partially ob- 

 scured by the papilla. The connection of the ovary with the oviduct is like a thin-walled, 

 broad, flat funnel which is no doubt made up largely of smooth muscle fibres and is 

 therefore very elastic. The tissues of the oviduct are continuous with those of the ovary, 

 its very much reduced lumen continuous with that of the ovarian lumen. The sole func- 

 tion of the oviduct is egg transport, or rather it is a muscular organ for extruding the 

 matured eggs. 



