The divisions that occur during segmentation are called cleavages, and the cells that form are called blastomers.

In the Animal Kingdom, the difference in the quantity and distribution of calf in the egg determines differences in segmentation, the lower the rate of division. Because of this, we can consider two basic types of segmentation:

  • holoblastic or total that occurs in the whole zygote;

  • meroblastic or partial, which occurs only in part of the egg.

Holoblastic Segmentation

Holoblast segmentation occurs in the alytes, isolites (or oligoliths) and heteroliths, and can be subdivided into three types based on the size of cells that form from the third cleavage (when changing the plane of cell division):

  • holoblastic equalin which the third cleavage forms eight equal blastomers; Occurs in the alectites eggs and some oligoliths;

  • uneven holoblastic, in which, with the third cleavage, blastomers of different sizes are formed (four smaller: micromers; and four larger: macromers); Occurs in all heterolithite eggs and some oligoliths;

  • holoblastics subequal, a type of uneven segmentation in which blastomers do not differ much in size, occurs in some isolated eggs.