Dev Growth Differ 1976 Jan 01;184:399-406. doi: 10.1111/j.1440-169X.1976.00399.x.

ELECTRON MICROSCOPIC OBSERVATIONS ON THE EXTRA-GRANULAR ZONE OF THE EMBRYO OF THE SEA URCHIN, HEMICENTROTUS PULCHERRIMUS.

Abstract
"Extra-granular zone" proposed by Dan and Dan (3) was studied with the electron microscope. Two stages are noticed during the process of its formation; at first, a relatively yolk granule-free zone constitutes the egg periphery which should correspond to that observed by Dan and Dan, and later, this zone is occupied by a layer of the vacuoles (about 1.3 μm in mean diameter) with highly electron-transparent inclusions which migrated to the egg surface from the inner cytoplasm after fertilization. These vacuoles are scarcely found in the micromeres of 16-cell stage embryo.

PubMed ID: 37281997
Article link: Dev Growth Differ