On theoretical grounds, one might expect to find a good deal of variation between cows in amounts of progesterone and oestrogen secreted by the ovary during the follicular phase of the oestrous cycle. Thus, there is variation in the rate of regression of corpora lutea (cl) in terms of size and progesterone content (Mares, Zimbelman & Casida, 1962; Niswender, Kaltenbach, Shumway, Wiltbank & Zimmerman, 1965), and limited data suggest that content is correlated with amounts of progesterone secreted into the ovarian vein (Gomes & Erb, 1965). Although little is known about oestrogen content of ovarian vein blood, considerable variation has been observed in numbers of Graafian follicles during the follicular phase (Rajakoski, 1960; Choudary, Gier & Marion, 1968; Lamond, unpublished). It seemed likely that variation in size of Graafian follicles might be associated with variation
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