Ovaries on Days 7 and 10.
The upper panel shows a sonogram of a Day 7 ovary, the lower panel a Day 10 ovary (of the estrous cycle). Each corpus luteum has a cavity, also called a lacuna, which is a normal fluid filled cavity. These are frequently referred to as "cystic" CLs, perhaps an unfortunate term because they are normal functioning corpora lutea secreting the same amount of progesterone as a solid CL, hence no treatment is indicated. On palpation these hollow, or cavitated CL feel rounder and can be mistaken for a luteal cyst. They can be differentiated by determination of the progesterone concentration or by ultrasonography.
Pieterse MC (1999)