Just to add to the discussion. Different amphibians even closely related ones can have different numbers of membranes as well as different strengths of those membranes. Obligate egg feedering tadpoles do have some adaptations of thier mouth parts that allow them to access the interior of the feeder eggs when provided but we should also consider that it is possible that the feeder eggs are also adapted for the tadpoles to be able to consume them. I am unaware that any studies documenting this as of yet (if someone knows of a study I would love to have the reference). This has been reported anecdotally as there have been a variety of trials over the years between hobbyists and institutions (use of auratus eggs, eggs of leopard frogs, the eggs of Cynops ssp). In some of these the eggs were readily taken once the egg was removed from the membranes with decent growth and success (Cynops ssp, and if I remember correctly auratus), while others did not fare so well (leopard frog).
There are also some reports from institutions (but not recorded in the literature) that when necropsied the digestive tracts of the obligate egg feeders also contained other potential foods such as fruit flies. The level of nutrition that the tadpole may derive from such materials is unknown.
Some comments,
Ed
