However, some fish eggs have different shapes because of the different shapes of egg membranes.
The size of fish eggs varies with species. There are very small eggs with a diameter of only 0. 3~0。 5mm, such as tiger fish; There are also very large eggs, up to 220mm in diameter, such as mouse shark eggs; The egg diameter of most fish is 1 ~ 3 mm, the eggs of oviparous fish are smaller, and the eggs of viviparous and ovoviviparous fish are larger.
Fish eggs are highly specialized cells, but they have the structural pattern of general somatic cells, that is, they are composed of nucleus (egg nucleus), cytoplasm (egg plasma) and cell membrane (egg membrane). In order to meet the physiological needs of in vitro development, the eggs of oviparous fish contain a lot of nutrients-yolk, oil beads and protective egg membrane.
1. In a mature unfertilized egg, the egg nucleus is in a division stage rather than a resting stage. The nucleus is located in the cytoplasm of the egg below the fertilization hole, and its long axis is perpendicular to the plasma membrane of the egg. Haploid chromosomes are arranged on the equatorial plate of the spindle.
2. Ooplasm is the cytoplasm of the egg, which can be divided into two areas: the surface layer under the plasma membrane is cortical and gelatinous, and the rest is endosperm. Oocytes not only have the same organelles as somatic cells, but also have the unique structure of eggs, such as cortical granules, yolk, oil globules, embryonic substances, enzymes and hormones. Cortical granules, also known as cortical vesicles or vacuoles, are wrapped by membrane and contain mucopolysaccharide pellets. The number, size and arrangement of cortical particles vary with fish species. Yolk, also called trophoblast, is the energy source of embryo development. In chemical composition, it can be divided into carbohydrate egg yolk, fat egg yolk and protein egg yolk. The yolk of some fish is granular; However, the mature eggs of other fish fuse into yolk pieces. Oil ball, also called oil drop, is a special part of many marine teleost eggs, which contains neutral fat. It is not only the storage of nutrients in floating eggs, but also a float. The existence, quantity, diameter and color of oil balls can be used as an important classification feature to distinguish various fish eggs.
3. Egg membrane According to the source and formation mode of egg membrane, fish egg membrane can be divided into primary egg membrane, secondary egg membrane and tertiary egg membrane.