The Croonian Lecture, 1976 - Egg cytoplasm and gene control in development

Abstract

This article is concerned with how a fertilized egg develops into a complete individual. In nearly all animal species (the main exceptions being mammals), ferti­lized eggs develop entirely independently of their mother. Commonly, eggs are surrounded by layers of materials such as membranes, jelly layers, or a shell, which isolate the egg and developing embryo from their environment. Embryos remain inside these coverings until they hatch as a mobile, free-living larva capable of feeding itself. Such a stage is usually reached only a few days or even hours after fertilization. During this time, development appears to take place without any chemical or physical instructions from the embryo’s environment. Development involves the formation of hundreds or thousands of cells from a single fertilized egg cell, as well as the conversion of yolk, a food-reserve, into the numerous different kinds of proteins which make up the cells of a complete larva. The question of how a superficially structureless egg converts itself, in a relatively short time, into a complex and highly organized structure has interested scientists since the time of Aristotle, 2000 years ago. However, specific concepts or explanations of early development were not well formulated until the eighteenth century. In 1779, for example, Bonnet made the explicit proposal that in each egg is a miniature embryo which itself contains an ovary with eggs, each of which themselves contain miniatures with ovaries, and eggs, and so on - the so-called doctrine of ‘emboitement’. Even Bonnet did not believe this doctrine in its strict sense, which would demand, as Bonnet’s own calculations showed, that Eve would have had 27 million embryos in her ovary. Throughout the nineteenth century there was extensive discussion of the relative merits of epigenesis and preformation. † In the later part of the nineteenth century, there arose the concept of neopreformation, according to which the preformed components of a fertilized egg were thought of as molecules and not morphological structures. For example, Lankester (1877) stated that: ‘Though the substance of a cell may appear homo­geneous under the most powerful microscope, excepting for the fine granular matter suspended in it, it is quite possible, indeed certain, that it may contain,already formed and individualised, various kinds of physiological molecules. The visible process of segregation is only the sequel of a differentiation already estab­lished, and not visible.’ This concept of the existence of determinant molecules in eggs may be taken as the point of departure for the present article. A concise account of early theories of development is included in Davidson’s (1968) book, and a history of embryology has been published by Needham (1934) and Oppenheimer (1955).