EARLY EVENTS LEADING TO FATE DECISIONS DURING LEECH EMBRYOGENESIS

This paper reviews leech development up to the 12-cell embryo. Oogenes is proceeds by a system of nurse cells that contribute to oocyte growt h via continuous cytoplasmic connections. Development begins when fert ilized eggs are deposited: formation of the polar bodies, and centrati on of the male and female pro-nuclei is accompanied by cytoskeletal co ntractions, and formation of teloplasm (yolk-free cytoplasm). The firs t cleavages are asymmetric: cell D' the largest macromere in the eight -cell embryo, contains most of the teloplasm. At fourth cleavage D' di vides equally; its animal and vegetal daughters are precursors of segm ental ectoderm and mesoderm, respectively. Teloplasm is a determinant of the D' cell fate. The expression pattern of Hro-nos, a leech homolo g to the Drosophila gene nanos, suggests that it may be a determinant associated with the animal sorter and inducing the ectodermal fate in the animal daughter cell of the D' macromere.