362 FISH FOR EXPERIMENTATION 



Fertilization of the egg occurs during or immediately after oviposition. Copulation takes 

 20-90 seconds, is preceded by a brief courtship and is accomplished by the male holding 

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

 pectoral fin. Since Oryzias is polygamous, best results are obtained in tanks of 30-50 

 fish in which there is an excess 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 Oryzias is small (1. 27 mm. in diameter), transparent, and possesses a thick 

 (0. 032 mm. ) membrane between which is a perivitelline space filled with a fluid. The 

 membrane is traversed by 8 striations parallel to the surface, and also 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 3 segments, found every- 

 where but at the vegetal pole. At this pole there are many longer (0. 95 mm. ) delicate 

 and sticky filaments, 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 appears 

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

 globules may be seen between the yolk and the periblast. During early development these 

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

 pole. This oil globule is gradually used up as nutritive substance and disappears simul- 

 taneously with the disappearance 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 

 oviparous fresh-water Cyprinodont, Oryzias (Aplocheilus) latipes, called the 

 "Medaka" in Japan where it is found. 



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

 velopment not customarily studied in embryology courses, material suitable for 

 laboratory experimentation. 



MATERIALS: 



Biological: Pairs of mature Medakas (Oryzias latipes). 



Plant material for aeration: Nitella, Utricularia, Elodea, Cabomba. 

 Food: Living Tubifex or Enchytrae. 



Dried shrimp, dried Daphnia, small-grained mixed dry fish food. 



Technical: Aquaria: 1, 5, 10 gallon capacities. 



Fish bowls: Ten cent store variety for pairs of fishes. Wire covers. 

 Celluloid cups for feeding living worms to fish. 

 Feeding ring for dried food. 



Aeration: Stream of air bubbles (not necessary if plants are abundant). 

 Artificial lighting: Gooseneck lamps with 25 watt bulbs if daylight is 

 inadequate or irregular. 



* The author acknowledges, with appreciation, the help of Dr< A. N. Solberg, Dr. A. J. Waterman, Mr. L. Roth and Mr. E. G. 

 Robinson In organizing this exercise. (See new references by Briggs and by Egami) 



