EXPERIMENTAL FISH EMBRYOLOGY 379 



Fourth day : Embryonic movementa begin; the blood is red; somites number 50-58. 

 Sixth day : Numerous chromatophores appear, hatching may occur on the 6th day, 

 depending upon temperature. In the hatching process the tail moves violently to 

 break the membrane, and the tail region emerges first. The size schedule is as 

 follows: 



Just hatched: h.3 - 5.0 mm. in length. 

 One month: 11.5 " 15.0 mm. in length. 



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

 Generally 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°-59°C. the optimum for all stages is between 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, h pints glacial acetic, 6 pints glycerine, and 85 pints 

 water) and after 5 days may be transferred permanently to 10^ formalin. Sectioning 

 la difficult but can be accomplished if the chorion is removed after fixation. 



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. 



NaCl 1.0 gram 



KCl 0.05 grams 



CaClg 0.05 grams 



MgSOij. 0.08 grams 



Distilled Water to 1 liter 



OBSERVATION AND TABULATION OF DATA : 



It is Important that the student become thoroughly acquainted with the reproductive 



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

 done (see Robinson and Rugh, 19'*-5). 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 2-5 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 : neuromeres; fore, mid, and hind-brain; 



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



pit and otocyst. 

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



development of blood vessels including sketches of the circulation on the Uth 



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

 k. Numerous wandering mesenchyme cells found toward the posterior end in 2-5 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 ( Aplochellxis) latipes, the Japanese Medaka (also known as the Geisha Girl 

 fish) has been adapted so satisfactorily to laboratory conditions that it promises to con- 

 tribute much to the field of experimental embiyology. For this reason a rather thorough 

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



