578 EXPERIMENTAL FISH EMBRYOLOGY 



EARLY DEVELOPMEMT OF OBYZIAS LATIPES : 



In an aquarium containing several dozen fish you will find eggs (I-80 per female) 

 every morning. Since the eggs are laid shortly after daylight these ohservations must he 

 made very early in the day. 



Using a dip-net, catch a female which has a cluster of eggs attached and transfer it 

 to a finger howl of aquarium water. Pick off the eggs either with a wide-mouthed pipette 

 or a pair of forceps. 'Egga which appear to be opaque within 10 minutes after removal are 

 very likely to be dead. Separate the eggs so that there are about 10-20 per finger bowl. 

 Keep no vegetation with the eggs. 



Observations should be made under dissecting microscope (mag. x 1? or more) with both 

 direct and transmitted lighting. Since the development is rapid (see diagrams and photo- 

 graphs by Eoth) and hatching generally occurs within 6 days, it will be necessary to make 

 rather constant observations or to supplement any series with observations on other eggs. 

 The schedule of development follows: 



TIME SCHEDULE OF DEVELOPMENT : 



1. Egg becomes translucent , perivitelllng space widens, germinal disc becomes lens- 



shaped: 10 minutes after fertilization. 



2. First cleavage : 1 hour after oviposition; cleavage meridional, nuclei visible. 

 5. Second cleavage : I5 hours, meridional, right angles to first; blastomeres equal. 



4. Third cleavage : 2 hours, plane parallel to first cleavage; blastodisc appears 



rectangular in outline with no space beneath. 



5. Fourth cleavage : 3 hours, parallel to the second; central blastomeres smaller 



than peripheral ones. Note margins of boundary cells. Is there any incorpora- 

 tion of yolk? 



6. Fifth cleavage : Js hours, unlike most teleost eggs, this cleavage does not give 



rise to layers of cells. 



7. Sixth cleavage : h hours, after which the cleavages are no longer synchronous but 



give rise to a many-layered blastodisc. 



8. Blastula : Ely 6-8 hours the sub-germinal cavity first appears. The blastodisc is 



elevated and a thickened marginal periblast of syncital nature may be seen. Ob- 

 serve in optical section if possible. Coinpare the periblast nuclei with those in 

 the center of the blastodisc in regard to shape and color. Ely 2k hours there is 

 nucleation of these periblast cells and the marginal thickening is accentuated 

 to become the germ ring. This germ ring will grow down over the yolk in much the 

 same manner as a bathing cap is pulled down over the head. 



9. Gastrula : It ia difficult to designate a specific time when gastrulation begins 



but it is accomplished by invagination, accompanied by delaminatlon and over- 

 growth at the time the germ ring has grown around about a third of the yolk mass. 

 The blastopore is represented as the uncovered portion of the yolk mass and 

 embryonic structures first appear with the closure of this blastopore. 

 10. Embryonic development : 



First day : The embryonic shield appears as a result of the thickening of the cells 

 along a region which represents the axis of the future embryo. This appears dur- 

 ing gastrulation as a result of migration of cells and will extend toward the 

 center of the blastodisc from a point along its margin. This embryonic shield 

 should be observed from all aspects. If possible, determine the regions of most 

 active growth and the source of cells that go into the various parts of the em- 

 bryonic shield. During the first 2U hours the embryo will encircle 5/8 of the 

 yolk mass, the optic vesicles will appear and there will be fewer than 10 somites. 



Second day : The embryo extends 5/8 around the yolk; the heart beats about 5O-60 

 times per minute; the blood is colorless; Kupfer's vesicle is seen at the caudal 

 end; and there may be 18 somites. 



Third day : The embryo extends ^/h around the yolk; the heart is beating as in the 

 adult, about 150-170 per minute; the blood Is slightly pink In color; pigment ap- 

 pears in the optic cup; and there are 20-28 somites. 



