FISH FOR EXPERIMENTATION 367 



Oryzias latipes reaches maturity in I-I2 months, and the life span is 1-2 years. Gen- 

 erally a fish hatched in one summer dies after it breeds during the following summer. 

 The life span of laboratory fed fish has not been determined. 



Care of Material: Avoid crowding, and observe eggs without undue handling. They 

 may be left in Stenders or finger bowls throughout development. When the fry 

 hatch they should be fed #0 (finest grain) baby fish food and protozoa. While the 

 range of temperature tolerance is 7 -39°C. the optimum for all stages is be- 

 tween 20°-25°C. In studying the normal series, use a constant and recorded 

 temperature and compare with the accompanying drawings and photographs. 



Fixation of any fish embryonic material may be in Bouin-Dioxan or in Stockard's 

 solution (5 pints formalin, 4 pints glacial acetic, 6 pints glycerine, and 85 pints 

 water) and after 3 days may be transferred permanently to 10% formalin. Sec- 

 tioning is difficult but can be accomplished if the chorion is removed after fixa- 

 tion. 



Note: A synthetic medium of balanced chemicals may be used for Oryzias if 

 conditioned tap water proves to be deleterious. This solution will also inhibit 

 the growth of molds and bacteria. 



OBSERVATION AND TABULATION OF DATA: 



It is important that the student become thoroughly acquainted with the reproductive phys- 

 iology of the fish particularly since there are very few fish with which this can be done 

 (see Robinson and Rugh, 1943). Compare any observations with those on the frog. 



The observations on early embryonic development are to be made under low magnifica- 

 tion almost continuously during the first 2-3 hours, then several times daily thereafter 

 until the embryo hatches (6th day). Note particularly the following: 



1. Somites : number as criterion of age; manner of formation; positional relation 

 to future parts of the central nervous system. 



2. Development of the central nervous system: neuronneres; fore, mid, and hind- 

 brain; cerebrum, cerebellum, medulla and cord; optic vesicle, lobe and lens; 

 olfactory pit and otocyst. 



3. Circulatory system: appearance and shape of the heart; the initial heart beat; 

 development of blood vessels including sketches of the circulation on the 4th and 

 6th days; changes in blood color from day to day. 



4. Numerous wandering mesenchyme cells found toward the posterior end in 2-3 

 day old embryos; observe movement and mitosis. 



5. Chromatophores: appearance, behavior, function; color and shape, contractility; 

 response to light. 



6. Movement: blastodisc; body; and finally the fins. 



7. Fry: behavior in response to various types of stimuli. 



DISCUSSION: 



Oryzias ( Aplocheilus) latipes, the Japanese Medaka (also known as the Geisha Girl fish) 

 has been adapted so satisfactorily to laboratory conditions that it promises to contribute 

 much to the field of experimental embryology. For this reason a rather thorough des- 

 cription of the adult fish, the reproductive physiology, and the egg will be given here. 



