zona pellucida

Just finished the histology lab exam and I did reasonably well. As always when I am given a picture and asked to produce words, I blank out on one or two. This time I blanked out "zona pellucida" which I know well and have studied. I know the location and function and duration of it, but I still could not dredge the words out of my memory for the exam. The ZP forms around an oocyte in the ovary when it matures from a primary to secondary follicle, and serves later to prevent double fertilization. When the head of the sperm dumps its chemicles into an oocyte, the zona pellucida immediately changes to prevent any more sperm from getting in.... The rest of the words dredged up OK tho, so I'm sure I passed.
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