The relationship between gametogenesis and conceptus development has been described in relation to recent experiments concerning selected aspects of the cellular and molecular basis of differentiation of female and male gametes. Evidence suggests that a variety of hormonal and nonhormonal mechanisms or combinations thereof have evolved and control distinct stages of oocyte maturation process. Analyses of the meiotic maturation process with particular emphasis on amphibians, indicate that distinct cytoplasmic factors are produced or activated in response to a hormonal stimulus which initiates germinal vesicle breakdown and reinitiates the meiotic maturation process. Evidence suggests that certain cytoplasmic factors originate in the nucleus and (or) cytoplasm and that considerable cytoplasmic maturation proceeds in the absence of the nucleus, or as a result of nuclear-cytoplasmic interactions. Cytoplasmic factor(s) or activities affect many aspects of oocyte function and structure, including the cell membrane, chromosomes, nucleus and meiotic spindle. Ions also play a crucial role in gamete differentiation either alone or in combination with hormones or these cytoplasmic factors. The relevance of these findings to oocyte maturation and fertilization, activation and embryonic development is discussed. In many cases, similar types of biological activities exist in widely separated species and (or) are effective in widely different species. The importance of synchronization of normal gametes to the development of the embryos is discussed. It is proposed, and the evidence suggests, that hormones, ions and cytoplasmic factors play a fundamental and variable role in the differentiation and function of "fully" mature oocytes of many species including mammals. Significantly, the data suggest that a "fully" mature oocyte occurs as a result of the expression of an underlying developmental program. Variations in these substances or the processes involved in their formation or actions appear to be relevant to understanding a wide variety of developmental abnormalities as well as to assessing the normality or abnormality of in vivo or in vitro maturation and differentiation.