FISH FOR EXPERIMENTATION 369 



h. The Platy, Platypoecilus Maculatus 



THE MULTI-COLORED PLATY; (XIPHOPHORUS) 



These live bearing Poeciliid fish are found in Mexico in the vicinity of the Rio Papoloapan 

 and Rio Panuco where the waters are slightly alkaline (pH tolerance of the fish is 6. 4 - 

 8. 0) and the temperature variable (temperature tolerance 65 F. to 90°F. with optimum 

 at 75°F. ). Dr. Myron Gordon of the N. Y. Aquarium has collected many specimens on 

 expeditions to Mexico and has distributed them freely for research purposes. 



These fish breed frequently and may drop as many as 80 young at a time. They are less 

 apt to devour their young than are the Guppies, but should be separated from their young 

 shortly after birth. The best aquarium temperature for breeding is 75 F. and the water 

 should be very slightly alkaline. Food is alternated between Tubifex or Enchytrea and 

 mixed dry granular food. The fish may be kept in colonies but gravid females should be 

 isolated just before dropping their young. 



The two species, P. maculatus and P. variatus, are rather common now, the former be- 

 ing so variable in color that there are hardly two fish with similar markings. The major 

 types are (1) Greyish blue with crescent markings; (2) Redish brown with speckles; 

 (3) Dark fish with black stripes and some black scales. The Gold Platy is a mutation in 

 which the black pigment has been lost and the yellow xanthine base has been exposed. 

 The colors are no bar to crosses, the varieties and species interbreeding freely. In fact 

 the Platy may be crossed with the Swordtails. There is now on the market a Platy-Sword- 

 tail hybrid known as the Montezuma Swordtail and another, the Black Swordtail, which is 

 a hybrid between the Mexican Swordtail and the Black Platy. Dr. Gordon has worked out 

 the genetic story in considerable detail, linking it with the inheritance of a type of fish 

 (melanotic) cancer. The embryology of these fish has been worked out (See Tavolga & 

 Rugh, 1947). Combined with the known genetic make-up, embryological studies would be 

 highly instructive. (See particularly papers by Gordon. ) 



DEVELOPMENT OF THE PLATY FISH, XIPHOPHORUS 

 (PLATYPOECILUS) MACULATUS 



DESCRIPTION OF NORMAL DEVELOPMENT STAGES 



Stage 1. Mature Ovum (Fig. 1) 



The mature infertile ova, after the yolk has been deposited, average 1. 5 mm. in diam- 

 eter. They are of a clear yellow color with peripherally arranged fat globules of various 

 sizes. These globules vary somewhat in size and number depending on the individual 

 ovary. However, the eggs in any one ovary are all very similar in this character. When 

 the egg is damaged, the globules are found to be adherent to the peripheral membrane; 

 they are composed of a colorless fluid somewhat less viscous than the yellow colored 

 matrix. 



The germinal vesicle cannot be seen in the living egg, but it has been demonstrated by 

 Hopper (I943) to be peripheral in position in sectioned material. 



The vitelline menibrane is probably present since a fertilization membrane is subse- 

 quently demonstrated. No tertiary membrane, such as is found in oviparous species, is 

 present around the egg. 



Immature eggs appear to be more opaque than mature fertilized ones. It may be that this 

 change takes place at fertilization as it does in Fundulus (Oppenheimer, 1937), but in this 

 viviparous species it is difficult to substantiate. 



