32 THE STRUCTURE OF THE FERTILISED EGG 



Therefore, the multiplicity of the egg cytoplasm must be mainly 

 intensive. 



For the sake of completeness, an alternative explanation of 

 the experiments described above must be mentioned. One can 

 still maintain that the egg cytoplasm has a complicated struc- 

 ture, which is disturbed in these experiments. In that case, the 

 existence must be postulated of a more or less mysterious 

 "vital force" which sees to it that the normal structure of the 

 egg is restored after the disturbance. Such an explanation was 

 given by Driesch, who regarded the result of his experiments 

 as proof of the existence of an entelechy which, after a disturb- 

 ance, "regulates" the course of development again. Undoubtedly, 

 this is a possible explanation, but it is not the most obvious 

 one because it is founded on the introduction of a new, and 

 entirely hypothetical, factor. Now it is the task of science 

 always to look for the simplest possible explanation, and to 

 accept this as long as it has not been disproved. Therefore, we 

 must reject Driesch's hypothesis so long as no more compelling 

 arguments can be advanced in favour of it, and we must accept 

 the conclusion that, on the whole, the egg cytoplasm is a homo- 

 geneous system with intensive multiplicity only. 



In the Introduction it was pointed out that the various animal 

 groups by no means behave identically in their development. 

 This applies also to the results of the experiments described 

 above. Often they show different results in other eggs than 

 those mentioned. Centrifuge experiments, for example, do in 

 fact cause disturbances in the development of the eggs of many 

 species. For some time past, therefore, two types of eggs were 

 distinguished, (1) "mosaic eggs" which possessed a complicated 

 spatial structure, and in which each disturbance of the system 

 resulted in disturbed development, and (2) "regulation eggs", 

 with a poorly developed, or highly plastic spatial structure, 

 in which disturbances of the system were easily "regulated". 

 This distinction, however, has proved unfounded. It has been 

 shown that the aberrant behaviour of „mosaic eggs" is not 

 due to fundamental differences in the structure of the egg, but 

 to a number of adventitious phenomena (cf. p. 63). Therefore, 

 it can be said of these eggs as well that, at the beginning of 



