CHAPTER m 



The structure of the fertilised egg 



The structure of the egg at the starting point of its development 

 seems to be quite simple. But, as we have seen in the Intro- 

 duction, it is conceivable that this simplicity is only apparent. 

 The whole complicated spatial structure of the later organism 

 might already be present, in a not easily recognisable form, 

 in the egg. According to the preformation theory, which had 

 many supporters in the 17th and 18th centuries, the complete 

 young animal was already contained in the fertilised egg, in 

 the same way as a complete stalk with leaves, flowers, etc., 

 can be contained in the bud of a plant. The development of the 

 egg would be no more than the unfolding of this preformed 

 young germ. The opposite view was maintained by the theory 

 of epigenesis, which held that the embryo was not yet present, 

 as such, in the egg, but that it would arise as a new product 

 in the course of development. In the 19th century, a thorough 

 study of the developmental phenomena became possible, thanks 

 to the great progress in microscopic technique made in that 

 period. By this means, the dispute between preformation and 

 epigenesis was at that time settled in favour of the latter theory. 

 A few years after 1880, however, the issue was reopened, in a 

 different form, by Wilhelm Roux. It had indeed been shown that 

 the embryo as such is certainly not yet present in the fertilised 

 egg, but was it not possible that the "degree of multiplicity" 

 of the egg's spatial structure was almost, or quite as high as 

 that of the embryo ? The egg might contain, in an invisible form, 

 such a complex system of causal factors, that each of its parts 

 would be able, quite independently, to produce one definite 

 part of the embryo, and nothing else. This implied that each 

 part of the embryo would be preformed in one definite part of the 



