It is well known that the sequence of nuclear changes which takes place during the differentiation of the oocyte is interrupted for a relatively lengthy period towards the end of meiotic prophase. This generally occurs with the onset of the diplotene phase, although in the primordial oocytes of some mammals it is superseded by the dictyate phase (see Baker & Franchi, 1967b). The meiotic division is normally only resumed shortly before ovulation, which indicates that for a particular oocyte the `resting' period may last from a few weeks to many years (Brambell, 1956; Franchi, Mandl & Zuckerman, 1962).
On the basis of histological observations, however, certain authors have claimed that the oocytes in the cow `rest' in the pachytene stage (Henricson & Rajakoski, 1959; Rajakoski, 1965; Erickson, 1966). They consider that the diplotene stage does not appear until the