174 FISH HATCHERY MANAGEMENT 



the fish culturist in providing an optimum environment for egg develop- 

 ment. 



Egg Development 



During oogenesis, when an egg is being formed in the ovary, the egg's fu- 

 ture energy sources are protein and fat in the yolk material. At this early 

 stage, the egg is soft and low in water content, and may be quite adhesive. 



The ovum, or germ cell, is enclosed in a soft shell secreted by the ovari- 

 an tissue. This shell, or chorion, encloses a fluid-filled area called the 

 perivitelline space. An opening (the micropyle) provides an entryway for 

 the sperm. Inside the perivitelline space is a vitelline membrane; the yolk 

 is retained within this membrane (Figure 60). Trout eggs are adhesive 

 when first spawned because of water passing through the porous shell. This 

 process is called water- hardening, and when it is complete, the egg no 

 longer is sticky. The egg becomes turgid with water, and the shell is 

 separated from the yolk membrane by the perivitelline space filled with 

 fluid. This allows the yolk and germinal disc to rotate freely inside the egg, 

 with the disc always being in an upright position. 



The micropyle is open to permit entry of the sperm when the egg is first 

 spawned. As the egg water- hardens, the micropyle closes and there is no 



MICROPYLE 



VITELLINE 

 MEMBRANE 



YOLK 



GERMINAL DISC 



SHELL 



PERIVITELLINE 

 SPACE 



OIL DROPLET 



Figure 60. Diagrammatic section of a fertilized trout egg. (Source: Davis 1953.) 



