Cytoplasmic determinants are spread unevenly in the egg, and so when embryo starts forming (cells start dividing), the determinants are also unequally divided between cell. This unequal distribution later plays a major role in differentiation and gene regulation.
Protostomes undergo determinate cleavage. Their cells' fates are determined very early in development, whereas deuterostomes undergo indeterminate cleavage. So, if you take away a cell at early stage of development, it can be replaced.
Now if we compare the division of cytoplasmic determinants in protostomes and deuterostomes, what I said in the first paragraph seems true (to some extent) for only protostomes.
- Do deuterostomes' oocytes have even distribution of cytoplasmic determinants initially and divide unequally only after certain number of divisions?
- Are there any studies showing correlation between the stage at which determination happens in deuterostome and beginning of unequal division of determinants?