Dorland's Medical Dictionary
Ovum = "the female reproductive cell which, after fertilization, becomes a zygote that develops into a new member of the same species. Called also egg. 2. the human mature oocyte: a round cell about 0.1 mm in diameter, produced in the ovary, where there is deposited around it a noncellular covering (oolemma; zona pellucida; zona radiata). It consists of protoplasm that contains some yolk, enclosed by a thin cell wall (vitelline membrane). There is a large nucleus (germinal vesicle), within which is a nucleolus (germinal spot). 3. the term was formerly extended to include any early stage of the conceptus."
ovule = "1. the ovum within the ovarian (graafian) follicle. 2. any small, egglike structure. 3. the megasporangium enclosed within one or more integuments that, after fertilization, becomes a plant seed."

ovum = ovule

From a researcher friend of mine : 1st one is used in biology
second one is the common word.

I would chose the one corresponding to the context.

I THINK these are the more generic terms.

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Note added at 43 mins (2006-09-19 09:20:39 GMT)
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But they are, as far as I am aware, distinct terms and cannot be used interchangeably.

English term (edited): ovum / ovule

It's true, most educated or semi-educated people in the U.S. would tend to use the word 'ovum' more than they would use the word 'ovule' but, really, if it came to discussing such matters the vast majority of Americans would simply use 'egg' or 'eggs'. Not suggesting that this be used as a serious substitute in a scientific context, just letting you know that in most cases the average American would tend to use 'egg' first, 'ovum' second and 'ovule' as a distant third. HTH.