The eggs, or zygotes, can be classified using as criteria the quantity and form of distribution of yolk in these cells. The veal is the nutritive substance present in the egg that helps in the early embryo development.
O calf can be distributed throughout the egg or concentrated in poles. The animal pole is the one in which the nucleus and organelles are present. In the vegetable pole, only veal is observed. The distribution pattern of the calf is directly related to how the cleavage occurs.
→ Egg types regarding calf distribution
According to the calf distribution, we can classify the eggs into:
Alecitos: Alecitic eggs are those that do not have a yolk. As an example, we can mention placental mammals.
Oligolocytes: Oligolocyte eggs, also known as isolocytes, are those that have little yolk, distributed homogeneously. This type of egg is present in many invertebrates, such as annelids, flatworms, non-cephalopod molluscs and echinoderms, and in amphioxes and tunicates.
Mesolocytes: These eggs, also called heterolecitos, have a moderate amount of yolk. In these eggs, there is the highest concentration of veal in the region called the vegetable pole. Amphibians have this type of egg.
Telelects: These eggs, also called megalecites, have a large amount of yolk, which takes up almost the entire cell. The nucleus and organelles are restricted to a small disc very close to the plasma membrane, called the germinal disc. Cephalopod molluscs, bony fish, reptiles, birds and non-placental mammals have this type of egg.
Centrolécitos: They have a large amount of calf, which takes up most of the egg, concentrating in the center. This type is found in arthropods, particularly insects.
→ Cleavage
O segmentation type that occurs in eggs is directly related to the amount of yolk present. The smaller the amount of veal, the more homogeneous the division. A larger amount of calf ends up making cleavage difficult, and this division will be more heterogeneous.
By Ma. Vanessa Sardinha dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/biologia/ovulos-animais.htm