EXPERIMENTAL FISH EMBRYOLOGY 371 



THE ZEBRA FISH EGG* 



The egg Is fertilized as it is laid. It is white, nearly opaque, and homogeneously 

 filled with granules. Its original diameter is 0.625 nun. and increases within a minute to 

 0.750 mm. and then shrinks to about 0.590 mm. after 2 minutes. This swelling of the mem- 

 brane and shrinking of the egg is associated with the formation of the perl vitelline space. 

 The membrane is soft and extensible. 



The protoplasm at oviposition is slightly concentrated, the egg as a whole becomes 

 oval and acquires a polar axis. The cell nuclei are visible after the first cleavage even 

 in the living egg. The schedule of early development at 27°C. follows: 



right angles to the first. 



parallel to the first, 



parallel to the second, 



parallel to the first, 

 horizontal. 



These time intervals are approximate, and at 25°C. the cleavage intei-vals are from 17-20 

 minutes after the first cleavage which is slightly delayed. The 10th cleavage is attained 

 in 3 hours at 27°C. 



Fixation is in Bouln or Bouin-Dloxan mixtures, with dehydration In Dloxan plus benzol. 

 The membrane and yolk may be removed after fixation. The best stains are the haemotoxy- 



lins. 



THE JAPANESE MEDAKA, ORYZIAS LATIPES 



These fish may be kept in groups of 50 in ten gallon tanks, and in such a colony egg 

 layers will be found almost dally throughout the year. The ratio of males to females 

 should be about 2/1. Breeding females may be Isolated with (2) males in small half- 

 gallon fish bowls, covered with wire screen to keep them from Jiunping out. Vegetation 



* This description is based largely on the work of Dr. Eoosen-Runge. He has worked with 

 the zebra fish, Brachydanlo rerio, more than any other Investigator, believes that there 

 is no fish to compare with it for embryologlcal studies. The eggs can be obtained at 

 all times and in great quantities if the following conditions are met: The fish are 

 kept in schools of 12 to I8 specimens, of which two thirds are males, and at a tempera- 

 ture of 26° to 29°C. The tanks should be from 8 to I5 gallon capacity. The fish are 

 fed once dally with mixed dry food, and occasionally with small pieces of (rat) liver. 

 Spawning occurs almost daily, the fertilized eggs sinking to the bottom where there 

 should be marbles or coarse gravel to hide the eggs from the fish. Eggs are removed 

 immediately with large pipette to finger bowls where they will develop very rapidly 

 (cleavage on an average of 18 minutes, gastrulatlon in 12 hours). The water in the 

 finger bowl should be changed daily. The eggs do not tolerate well the lower tempera- 

 tures . 



The egg is not sticky. Is nearly transparent, develops pigment late, has no dis- 

 turbing oil drops in the yolk, and develops so rapidly that the germ ring and the clo- 

 sure of the blastopore can be obsei-ved during the course of a single laboratory period. 

 A lOx objective (total magnification lOOx) shows the fine cellular detail and permits 

 observation of the living nuclei. The egg lies naturally on its side, so that a profile 

 view of the blastodisc and yolk is Inevitable when viewed through the microscope. Its 

 use in operational procedures has not yet been adequately tested. (See Eoosen-Runge, 

 1936: Anat. Anz. 81:297; 1938, Biol. Bull. 7'*:119; 1939, Biol. Bull. 77:79; 1959, Anat. 

 Bee. 7''-:'*39 for details. A motion picture of the development of this egg Is available 

 through The Vflstar Institute.) 



