In most species, eggs are much larger than sperm. Egg cytoplasm is well stocked with organelles, nutrients, and a variety of molecules, including cytoplasmic determinants such as transcription factors and mRNAs (see Key Concept 19.2). Cytoplasmic determinants in the egg play important roles in setting up the signaling cascades that orchestrate the major events of development: determination, differentiation, morphogenesis, and growth. Nearly everything the embryo needs during its early stages of development comes from the mother, including its mitochondria (and therefore all of its mitochondrial DNA).

In addition to its haploid nucleus, the sperm makes one other crucial contribution to the zygote in most species—the centriole. The centriole contributes to the zygote’s centrosome, which organizes the mitotic spindles for subsequent cell divisions (see Figure 11.9). Centrioles are also the origin of the microtubules of the primary cilia, which are important in cell signaling, as you will see when we return to the opening question about how embryos know left from right.