During gastrulation, cells containing different maternal factors come to be adjacent to one another. During gastrulation, there is little or no expression of the embryo's genome. They are highly conserved and have been found in animals as diverse as insects and humans. They encode transcription activators. The maternal contribution is greater than the paternal.
Homeotic genes determine segment morphology.
Neurulation occurs before gastrulation. As blastulation proceeds, more and more but smaller and smaller cells are produced. In mammals, spermatogenesis produces four haploid spermatids from the precursor cell. Blastulation in birds and frogs differs considerably. The acrosomal vesicle, which contains digestive enzymes needed for fertilization, is part of the egg.