Cell determination is a process through which portions of embryonic genome are selected for expression in particular embryonic cells. Determination to follow a specific pathway of differentiation precedes the appearance of any type of visible change in biochemistry or morphology of the cell. We know of two major ways through which determination takes place. in one way certain factors that reside in the cytoplasm of embryonic cells (intrinsic factors) control cell determination. These factors are considered to as ooplasmic or cytoplasmic determinants. During cleavage they are passed on to particular blastomeres whose future differentiation is so finally decided. This process is also called determination by cytoplasmic segregation.
In the other method, the final cell fate is determined through extrinsic factors that originate outside the cells. The extrinsic factors include:
1) The instructions acquired as a function of the position of blastomeres within the embryo,
2) Signals that are transmitted among the blastomeres.
This second process is termed as embryonic induction in which the cell fate is determined by signals received from different cells. In different species determination takes place at different times during development. It may be very early or it may take place at relatively much later stages. In quite a number of animal groups, like annelids, molluscs, tunicates etc. the fates of the blastomeres resultant from the first few cleavage divisions are already determined. Such types of eggs and embryos are called determinate or mosaic. In others such as echinoderms and vertebrates final determination takes place much later, even as late as during or after gastrulation, and they are said to have indeterminate or regulative eggs.