366 FISH FOR EXPERIMENTATION 



Observations should be made under dissecting microscope (mag. x 17 or more) with both 

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

 graphs by Roth) 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, perivitelling space widens, germinal disc becomes lens- 

 shaped: 10 minutes after fertilization. 



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



3. Second cleavage: 1^ hours, meridional, right angles to first; blastomeres equal. 



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

 tangular 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 incorporation of yolk? 



6. Fifth cleavage: 3j hours, unlike most teleost eggs, this cleavage does not give rise 

 to layers of cells. 



7. Sixth cleavage: 4 hours, after which the cleavages are no longer synchronous but 

 give rise to a many-layered blastodisc. 



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

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

 in optical section if possible. Compare the periblast nuclei with those in the center 

 of the blastodisc in regard to shape and color. By 24 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 is difficult to designate a specific time when gastrulation begins but it is 

 accomplished by invagination, accompanied by delamination and overgrowth 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 embryonic 

 shield. During the first 24 hours the embryo will encircle 3/8 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 50-60 

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

 end; and there may be 1 8 somites. 



Third day: The embryo extends 3/4 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. 



Fourth day: Embryonic movements begin; the blood is red; somites number 30-38. 



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

 pending 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: 4. 5 - 5. mm. in length. 

 One month: 11.5- 13.0 mm. in length. 



