Cell surface differentiations during early embryonic development

Polarized site of sperm entrance in the egg of a freshwater bivalve, Unio elongatulus

We studied the organization of the egg of a freshwater bivalve, Unio elongatulus. This egg is markedly polarized. At the vegetal pole there is a crater which constitutes the point of attachment of the growing oocyte to the ovarian wall. This has previously been interpreted as a micropyle. We show that the sperm does not enter the egg through the crater but in a differentiated region around it, mostly at its base. This region is characterized by a wrinkled surface and is the only site of the vitelline coat which specifically binds the lectin from Lotus tetragonolobus. The egg reacts explosively upon fertilization, ejecting vacuolar material from the crater. The role of this "egg reaction" in relation to the prevention of polyspermy is discussed.

On the segregation of the germ and somatic cell lines in the embryo

In this paper we argue that the mechanisms underlying the segregation of the somatic and germ cell lines are basically similar. The interaction of the genome with specific cytoplasmic factors is responsible for the restriction of their developmental potencies in the somatic cell lines, and for the prevention of such a restriction in the germ cell line. In particular, it is suggested that meiosis is part of the differentiation program of the germ cells.