The tertiary follicle develops due to a combination of a further increase in size of the oocyte, the proliferation of follicular cell layers, and the formation of liquid spaces between the follicular cells. These spaces gradually coalesce to form a follicular cavity (antrum folliculare). The oocyte has now reached its full size; its diameter has enlarged from 10 ?m in the primordial follicle to about 100 ?m. The theca folliculi is subdivided into a theca interna (several layers of epithelioid cells), and a theca externa (layers of concentrically disposed myofibroblasts). Shortly before ovulation, the tertiary follicle develops into the mature follicle (Graafian-follicle, pre-ovulatory follicle). The follicular cavity increases in size, causing the follicular cell layers to flatten. However, a wreath of follicular cells remains around the oocyte, called the corona radiata. The oocyte and corona radiata together form the cumulus oophorus. The oocyte completes its 1st maturation division (Fertilization) before ovulation.