This is the first time I've ever seen one of these up close and personal so it's a real treat. The egg sac of this species is very distinctive among Japanese hynobids -- the only one to be milky. I was surprised at its consistency -- very soft and gooey (unlike H.tokyoensis egg sacs, for instance, which are surprisingly firm). But when it's put back into water, it naturally tightens up again.
For more pics of eggs, larvae, juvies and adults, see:
http://www.rieo.net/amph/saramand/sansyouo/kurosan.htm