37h EXPERIMENTAL FISH EMBRYOLOGY 



The maturati onal procesaea are apparently at their peak during the quleacent period 

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

 poaition occura. There ia no doubt that the light cycle in aome way regulatea the egg- 

 laying periods, although other factors such aa food, pH, and temperature cannot be ignored. 



The eggs are held in the ovarian lumen until all the matured ones have ovxilated and 

 ovipoaition of all of the egga occura during a very brief period. A single female may 

 produce from I-80 egga per day, the average being between 20-30. During a aingle season 

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



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

 tending from the posterior ventral margin of the ovary to the exterior Juat posterior to 

 the urlno-genital papilla. A ventral aspect of the female ahows thia opening partially 

 obscured 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 muacle fibrea 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 

 function of the oviduct ia egg tranaport, or rather it is a muscular organ for extruding 

 the matured egga. 



Fertilization of the egg occurs during or immediately after ovipoaition. Copulation 

 takes 20-90 seconds, ia preceded by a brief courtship and is accon5)li3hed by the male hold- 

 ing the female with its dorsal and anal fins while spreading the milt over the eggs with 

 its pectoral fin. Since Oryziaa is polygamous, best results are obtained in tanks of 

 50-50 fiah in which there is an exceas of males. The eggs remain attached to the female 

 for '4--5 hours or until they are brushed off by the vegetation or other objects. 



THE ORYZIAS EGG 



The egg of Oryziaa ia small (1.27 mm. in diameter), transparent, and posaesaea a 

 thick (0.052 mm.) membrane between which is a perivitelline space filled with a fluid. 

 The membrane is traversed by 8 atriations parallel to the surface, and alao by numerous 

 radial canaliculi. The outer surface of the membrane is wavy, the inner surface smooth. 

 There are scattered threads (0.025 mm. long) each possessing 5 segments, found everywhere 

 but at the vegetal pole. At thia pole there are many longer (0.95 mm.) delicate and 

 sticky filamenta, each with two less obvious segments. These filaments entangle the eggs 

 with other eggs, causing them to cluster. Both the chorion and the filaments are derived 

 from the follicle while the egg is still in the ovary. 



The egg possesses a thin protoplasmic layer around a yolk sphere, uniform except at 

 the poles and definitely thicker at the animal pole where the germ disc will form. Liquid 

 yolk fills the bulk of the egg, and is white or yellowish. In the ovary this yolk appeara 

 to be opaque but becomes translucent shortly after fertilization. At oviposition many oil 

 globules may be seen between the yolk and the perlblaat. During early development theae 

 decrease in number by confluence and merge into a single large globule at the vegetal pole. 

 Thia oil globule is gradually used up as nutritive substance and disappears simultaneous- 

 ly with the disappearEinoe of the yolk sac. 



THE BREEDING AND EARLY DEVELOPMENT OF ORYZIAS LATIPES* 



DEFINITION : A study of the normal reproductive processes and early development of the ovi- 

 parous freah-water Cyprinodont, Oryzias ( Aplochellus) latipes, called the "Medaka" in 

 Japan where it is found. 



PURPOSE : To acquaint the student with reproductive processes and a type of early develop- 

 ment not customarily studied in embryology courses, material suitable for laboratory 

 experimentation. 



The author acknowledges, with appreciation, the help of Dr. A, N. Solberg, Dr. A. J. 

 Waterman, Mr. L. Both and Mr. E. G. Eoblnson in orgeinizing this exercise. 



