nSH FOR EXPERIMENTATION 359 



THE ZEBRA FISH EGG* 



The egg is fertiUzed as it is laid. It is white, nearly opaque, and homogeneously filled 

 with granules. Its original diameter is 0. 625 mm. and increases within a minute to 

 0. 750 mm. and then shrinks to about 0. 590 mm. after 2 minutes. This swelling of the 

 membrane and shrinking of the egg is associated with the formation of the perivitelline 

 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: 



15 minutes - movement inside yolk. 



35 minutes - 1st. cleavage depression. 



40 minutes - 1st. cleavage completed. 



60 minutes - 2nd. cleavage depression, right angles to the first. 



67 minutes - 2nd. cleavage completed. 



92 minutes - 3rd. cleavage depression, parallel to the first. 



104 minutes - 4th. cleavage depression, parallel to the second. 



117 minutes - 5th. cleavage depression, parallel to the first. 



136-146 minutes - 6th. cleavage depression, horizontal. 



These time intervals are approximate, and at 25° C. the cleavage intervals 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 Bouin or Bouin-Dioxan mixtures, with dehydration in Dioxan plus benzol. 

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

 toxylins. 



e. The Jewel Fish, Hemichromus 



These fish are somewhat temperamental and randonn pairs will not necessarily be con- 

 genial. Combinations must be tested and when a congenial pair has been brought together 

 they should be left together permanently. 



The Jewel Fish is rather large and should be provided with a 15 gallon tank for each pair. 

 They are voracious eaters, consuming large numbers of Tubifex and much oatmeal- 

 shrimp mixture. The latter food is made by cooking oatmeal and adding to it dry shrimp 

 commonly used alone as fish food. The tank must be cleaned frequently. 



♦ This description is based largely on the work of Dr. Roosen-Runge. He has worked with the zebra fish, Brachydanio rerio, more 

 than any other investigator, believes that there is no fish to compare with it for embryological studies. The eggs can be obtained 

 at all times and in great quantities if the following conditions are met: The fish ore kept in schools of 12 to 18 specimens, of 

 which two thirds are males, and at a temperature of 26 to 29°C. The tanks should be from 8 to 15 gallon capacity. The fish 

 are fed once daily 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 re- 

 moved immediately with large pipette to finger bowls where they will develop very rapidly (cleavage on an average of 18 min- 

 utes, gastrulation in 12 hours). The water in the finger bowl should be changed daily. The eggs do not tolerate well the lower 

 temperatures. 



The egg is not sticky, is nearly transparent, develops pigment late, has no disturbing oil drops in the yolk, and develops so rapidly 

 that the germ ring and the closure of the blastopore can be observed during the course of a single laboratory period. A lOx ob- 

 jective (total magnification lOOx) shows the fine cellular detail and permits observation of the living nuclei. The egg lies natur- 

 ally 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 Roosen-Runge, 1936: Anat. Anz. 81:297j 1938, Biol. Bull. 

 74:119; 1939, Biol. Bull. 77:79; 1939, Anat. Rec. 74:439 for details. A motion picture of the development of this egg is avail- 

 able through The Wistar Institute. ) 



