Immature eggs are much smaller than ripe eggs and are usually nearly clear or opaque white or yellow, depending on the fish species. Eggs that have begun to resorb often appear soft or partially deflated rather than firm and round. This process is the result of coalescence of the oil droplets into several large droplets and then into a single oil globule. This clearing of the egg is used to estimate the time of ovulation. Only eggs of fish that are within 15 hours of ovulation can be accurately staged using this method.
Immature eggs appear much smaller and remain opaque following hormone injection.
Observing the position of the nucleus is a good method of determining egg development. The weight of the cover slip slightly compresses the egg. When lighted from below, the nucleus appears as an translucent circle in the opaque egg when viewed under a microscope or hand lens. However, if the nuclei of a great majority of the eggs in the sample are near the edge or off-center, there is a high probability that the eggs are ripe. Within 5 to 20 minutes in the solution, the nucleus of the egg can be seen.