ovum vs ovule

ovum and ovule both are nouns.

Word	Noun	Adjective	Verb	Adverb
ovum	Yes	No	No	No
ovule	Yes	No	No	No

As nouns, ovule is a hyponym of ovum; that is, ovule is a word with a more specific, narrower meaning than ovum:

Other hyponyms of ovum include egg, ootid.

ovum (noun)	ovule (noun)
the female reproductive cell; the female gamete	a small or immature ovum
	a small body that contains the female germ cell of a plant; develops into a seed after fertilization