Introduction

Abstract

The ontogeny of fishes starts with fertilization, or more precisely, with activation¹ of the egg. At the end of the endogenous food supply the major primary organs are usually established. In some fish this stage is attained in a few hours (e.g. zebrafish), in others it takes a few months (salmonids). Since temperature within certain limits enhances or reduces developmental time, so-called degree-days (temperature in °C multiplied by days) have been introduced for comparative analyses. However, this path did not lead to reliable answers. ‘Ignatieva (1976a)’ proposed a hypothesis that in early embryogenesis ‘an interaction between the yolk and active cytoplasm, which determines time of the onset of ribosomal RNA synthesis, is of importance in controlling the temporal relationships’.

‘Now is certainly the time to fish — not to cut bait.’

Detrich, Westerfield and Zon (1999)