Introduction and Notes on Materials and Methods
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The study of embryogenesis is increasingly concerned with the problems of the earliest stages of development. Mammalian ova, unlike those of lower vertebrates and invertebrates, have been largely neglected by researchers studying the problems of early embryogenesis, although they represent a unique object for investigation of the first differentiation processes. This is due to the fact that obtaining a great number of mammalian ova from the period of fertilization and, mainly, that of cleavage is connected with a number of technical difficulties which are further multiplied when the specimens are prepared for electron microscopy and ultracytochemistry. The possibilities for using segmenting mammalian ova in experimental studies are rather limited.