I think the "slimy stuff" was, as Knight of Gondor and Diamond18 have said, embryonic fluid, but I think it's purpose was for PJ to show that Saruman created the Uruk hai, and he did it at about this point in the books. The Uruks were Saruman's critters, as opposed to orcs, who, we are told, are elves who were long-ago corrupted by Morgoth. But the slime-pods were PJ's depiction/interpretation. Not JRRT's words.
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- Eve
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