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Fertilization is the union of sperm and egg that restores the diploid chromosome number and activates the developmental program. Species-specific surface molecules ensure recognition, and fast (membrane depolarization) and slow (cortical reaction) blocks to polyspermy prevent multiple sperm from fertilizing the same egg. Following fertilization, the zygote undergoes cleavage — rapid mitotic divisions that partition the large egg cell into progressively smaller blastomeres without increasing total embryo volume. Cleavage patterns (radial, spiral, bilateral) are species-specific and determined by the amount and distribution of yolk. Early cleavage establishes the basic cell number and spatial relationships that set the stage for gastrulation.
Development begins with a single event — the fusion of two specialized cells — and immediately faces two challenges: preventing additional sperm from entering, and converting one giant cell into many smaller ones that can begin the work of building an organism. Fertilization and cleavage solve these problems and set the stage for everything that follows.
Fertilization involves species-specific molecular recognition (sperm surface proteins binding egg coat receptors), followed by sperm-egg membrane fusion and activation of the egg's developmental program. The egg, which has been arrested in meiosis, completes its final meiotic division and initiates a cascade of intracellular calcium release. This calcium wave triggers the cortical reaction — exocytosis of cortical granules that modify the egg's extracellular coat into a hardened fertilization envelope, creating a permanent barrier to additional sperm. In many species, an earlier fast block (membrane depolarization) provides immediate, temporary polyspermy prevention while the cortical reaction is being assembled. Polyspermy must be absolutely prevented because it introduces extra chromosomes, which is invariably lethal.
Cleavage begins almost immediately after fertilization. The zygote divides rapidly — in some species, every 30 minutes — without any cell growth between divisions. These are stripped-down cell cycles consisting only of S phase and M phase, powered entirely by maternal mRNAs and proteins stockpiled during oogenesis. The result is progressive subdivision of the egg's cytoplasm into smaller and smaller blastomeres, eventually forming a hollow ball called the blastula (in many species). The cleavage pattern is profoundly influenced by yolk — a nutrient reserve that varies enormously across species. Sea urchin eggs have little yolk and cleave symmetrically. Frog eggs have moderate, vegetally concentrated yolk and cleave unequally (vegetal cells are larger). Bird eggs have so much yolk that cleavage is restricted to a tiny cap of cytoplasm on top.
Cleavage is not merely a subdivision exercise — it has developmental consequences. Different regions of the egg cytoplasm contain different maternal determinants (mRNAs, transcription factors, signaling molecules deposited during oogenesis). As cleavage partitions the cytoplasm, these determinants are distributed unequally among blastomeres, creating the first molecular differences between cells. In many organisms, the cleavage-stage embryo contains cells already biased toward different fates by their cytoplasmic inheritance. This sets up the initial asymmetries that the subsequent processes of gastrulation and induction will elaborate into the body plan.