It provides all of the nourishment for the developing embryo, diminishing in size as the embryo grows. In chickens, post-hatching nutrition is also mostly from the yolk for a short time, as it's not completely absorbed when the chick hatches out. To see the avian ovary click here. Each follicle is surrounded by a well developed system of blood vessels. At the time of ovulation of a follicle, a whitish area, the stigma , appears on the surface of the follicle, and the egg is expelled into the upper end of the reproductive tract.
Nothing resembling the mammalian corpus luteum is formed in birds.
These features can be made out on slides 221 and 680, and the variation in sizes among the follicles is a reflection of their different stages of development. Released eggs pass into the oviduct , also present only on the left side. The various divisions of the oviduct are all continuous with one another. The expanded upper end is called, as in mammals, the infundibulum , seen on slides 207 and 702. The infundibulum has fimbriae , finger-like projections covered with cilia, as in mammals.