Regional Morphological and Cytochemical Differentiation in the Fertilized Egg of Discoglossus pictus (Anura)

Abstract

A vertical column of cytoplasm poor in yolk (CPY) is located in the centre of the animal region of the unfertilised and fertilised egg of Discoglossus pictus. At the base of this column is found a central region of CPY designated as “clear cytoplasm”. Cytochemical methods show that the CPY in this whole region is rich in glycogen and RNA. By 60 min post opposition (p.o.) the pigmented cortical layer starts moving towards the future ventral side. It attains its definitive position by 90 min p.o. when the grey crescent, visible from 75 min p.o. onwards, achieves its maximal extension on the future dorsal side. The cytoplasmic column is now tilted towards the future ventral side. It progressively loses its direct contact with the cell membrane and disappears. From 90 min p.o. onwards, the “clear cytoplasm” is found progressively closer to the dorsal grey crescent cortex. When the first cleavage furrow appears at 135 min p.o., the “clear cytoplasm” is situated very near the dorsal cortex and even extends somewhat below the equator. In places a relatively thin layer of cytoplasm containing medium-sized and a few large yolk granules intervenes between the grey crescent cortex and the “clear cytoplasm”. These displacements suggest that sperm entry evokes a dorsally directed cytoplasmic movement in the animal half of the egg which, among other things, may facilitate an interaction between the vegetative yolk and the grey crescent cortex, or may directly influence the dorso-ventral polarisation of the vegetative yolk.