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Alternatively, it has been proposed that the ovarian medullar may produce a factor that retards follicle activation. Further insight into the phenomenon of spontaneous follicle activation has been generated from studies demonstrating that primordial follicles in bovine and baboon cortical explants grafted to chick embryonic membranes are not spontaneously recruited. These findings argue against the requirement for inhibitory factors originating from the ovarian stroma, or growing follicles, to prevent spontaneous activation. The grafted ovarian tissue was vascularized rapidly in ovo, and, consequently, the authors suggested that a factor of systemic origin may regulate global follicle recruitment. This study also suggests that in vitro conditions intrinsic to the culture system itself may in some way promote activation. In this regard, it has been suggested that in vitro spontaneous follicle activation is a consequence of exposure to an environment that is richer than the normal in vivo situation. Parrott and Skinner attributed some of the spontaneous activation of primordial follicles in in vitro-cultured whole rat ovaries to endogenous KITL.