Nearly all animals and plants that reproduce by asexual means also reproduce sexually at some stage. Stages of Development Fertilized eggs all develop in a similar pattern. Egg and sperm formation require meiotic cell divisions that separate the pairs of chromosomes. Fertilization is the penetration of the egg by a sperm. Only one sperm can enter and it must digest its way into the egg.
Cleavage refers to the egg dividing into numerous cells.
Each cell getting smaller as there is no new energy input. A third layer forms between the outside and inside layers. Already cells have differentiated into potential tissues called ectoderm that forms nerves and skin, mesoderm that forms muscle, bone, and connective tissue, and endoderm that forms the digestive tract and associated glands. Organ formation - cells are put together from all three layers to form organs as skin, heart, complete muscles, liver, pancreas, etc. Growth , Tissue Specialization - organs grow and their cells become more specialized and functional.