THE STRUCTURE AND DEVELOPMENT OF THE EXTERNAL MEMBRANE IN YOUNG EGGS OF THE BROOK TROUT, SALVELINUS FONTINALIS (MITCHILL)

Abstract

A study of the external membranes of the developing oocyte of the brook trout, Salvelinus fontinalis, was made by means of electron microscopy. The membrane, zona radiata, which becomes "hardened" in mature eggs after they are shed into water, was observed to begin development at the bases of microvilli which project from the surface of the oocyte. This membrane grows until in the mature egg it is about 50 μ thick. The zona radiata is completely permeated by numerous pore canals. In immature oocytes, the pore canals contain microvilli which arise from the surface of the oocyte. The microvilli make contact with the follicular cells surrounding the developing oocyte. Morphological changes which occur in the zona radiata and the other layers of developing oocytes are described at several stages of development. The relationship between morphological changes in the membranes and the transport of nutrients to the developing oocyte is discussed.The nomenclature of the membranes of the mature trout egg is discussed in relation to the findings of the present study. It is concluded that primary membranes and perhaps secondary membranes are present in the mature trout egg.