Small non-atretic follicles (1·5–250 mg) were collected from four distinct areas of the ovary of the laying hen. These were as follows: (1) the stalks of the two largest preovulatory follicles; (2) the stalks of the third largest preovulatory follicle; (3) the stalks of the fourth and fifth largest preovulatory follicle and (4) zona parenchymatosa.
There was an increase in the proportion of small follicles in the size range 1·5–10 mg collected from the stalks of the large yellow yolky preovulatory follicles approaching ovulation, with a corresponding decrease in the number of small follicles in the size group 10–250 mg. In addition, the ornithine decarboxylase activity in follicles collected from regions 1 and 2 was significantly greater than that in small follicles collected from region 4.
Since the level of ornithine decarboxylase activity is critically dependent on the degree of hormonal stimulation and growth rate of the tissue, it is suggested, on the basis of the differences in their ornithine decarboxylase activity, that small follicles located on the stalks of the large yolky preovulatory follicles are stimulated by trophic hormones and growth factors to a greater degree than similarly sized follicles located elsewhere in the ovary. It is proposed that this increased stimulation may increase the chances of these small follicles being recruited into the hierarchy.