Slugs and infertile eggs aren't the same thing slugs are follicular that never developed to the point of being viable. From what I am hearing and reading slugs are the girls fault, either she got to hot or to cold ect ect. Infertile eggs are from my limited experience the males fault if you get a perfectly formed filled egg simply missing the embryo the the female did everything except fertilize the egg. this could be due to dead genetic material from the male, not enough, or simply bad..