POSTFERTILIZATION CHANGES IN DISCOGLOSSUS-PICTUS (ANURA) EGGS RESULT IN THE FORMATION OF A CAPSULAR CHAMBER WHERE THE EGG ROTATES

Discoglossus pictus is one of the few anurans with an egg where a capsular chamber forms as a consequence of fertilization; the egg with its vitelline envelope rotates in this chamber according to gravity. We investigated the formation of the capsular chamber through various experimental cytochemical and ultrastructural approaches, and found that it is the product of plug liquefaction. The plug is a lens-shaped jelly coat typical of Discoglossus, and covering only part of the egg animal half. About 15 min after fertilization, granular material coming from the egg enters the plug, which gradually dissolves and, once liquefied, reorganizes itself around the entire egg, thus forming the chamber. This process goes through stages of rearrangement of the 25-angstrom- and 250-angstrom-thick filaments which constitute the plug matrix. The material entering the plug derives from the exocytosis of two vacuole types, with electron transparent and granular PAS-positive contents. Liquefaction of the plug correlates with the reduction of disulfide bonds present in its matrix. Furthermore, in vitro tests showed that the substances released from the egg are active in selectively dissolving only the plug, and lose activity upon boiling.