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H. D. McREYNOLDS and R. HADEK

Pyruvate is the central energy source for the developing mouse oocyte and zygote (Biggers, Whittingham & Donahue, 1967) and for the two-cell stage embryo (Brinster, 1965), but glucose can be utilized as the sole energy source beginning at the eight-cell stage (Brinster & Thomson, 1966). A histochemical study by Thomson & Brinster (1966) has shown that glycogen is present in the ovulated mouse oocyte and increases in concentration until the blastocyst stage, when it almost completely disappears. Hence, these authors suggested that the polysaccharide is an energy source for the developing embryo. Since whole-mount preparations were employed in the histochemical study, and because the embryo enlarges between the 4th and 5th day, Ozias & Weitlauf (1971) suggested that the total

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H. D. McREYNOLDS and R. HADEK

Numerous studies have been published on the ultrastructural aspects of mammalian fertilization (Szollosi & Ris, 1961; Hadek, 1963, 1969; Austin, 1968; Barros & Franklin, 1968), and several recent articles have described changes in the head of the mammalian spermatozoon following its incorporation into the egg (Szollosi, 1965, 1969; Bedford, 1968; Hadek, 1969; Presley, 1969). Little attention, however, has been paid to the fate of the sperm tail in the cleaving ovum. Although the axial filaments of the fertilizing mouse spermatozoon can still be observed within the zygote a few hours after copulation (Presley, 1969), swelling and breakdown of the paternal mitochondria occur before the first cleavage, and the sperm tail disintegrates soon thereafter (Szollosi, 1969). Sperm tails (assumed to be supernumerary) have nevertheless been