This is a cluster of eggs of a land slug Arion obesoductus.
It was laid by a specimen, that I captured in central Moravia at October 26, 2017. I captured only one slug and it later laid this cluster of four eggs in captivity. At least two slugs successfully developed and hatched out from these eggs.
I am not sure if there are any informations available about the life cycle of Arion obesoductus. I do not know if laying of four eggs is a rule or exception. Arion obesoductus is a small slug with a maximum body length of 25 mm. So laying clusters with small number of eggs makes sense. Otherwise these eggs looks like any other eggs of slugs from the Arionidae family as well as similar to eggs of many other land gastropods.
The cluster of eggs at December 13, 2017:
The same cluster of eggs at December 18, 2017:
References
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gastropods 