THE STRUCTURE OF THE FERTILISED EGG 31 



It becomes even less tenable in the light of the following facts. 

 A profound disturbance of the structure of the egg cytoplasm 

 often does not result in aberrant development. Such a disturb- 

 ance can be caused by centrifuging the egg. Under the in- 

 fluence of the centrifugal force, the materials of the egg are 

 arranged according to their specific gravity. The lighter 

 substances (especially the fats) are accumulated at the centri- 

 petal pole, the heavier material moves toward the centrifugal 

 pole. In this way the contents of the egg are arranged in layers, 

 an arrangement very different from the normal structure of 

 the egg cytoplasm (Plate III). Nevertheless normal development 

 will often take place in such centrifuged eggs, e.g. in polychaete 

 worms and in molluscs. In the course of this development, the 

 stratification is lost to some extent, and the material of the 

 egg is redistributed more evenly. Finally, normal embryos will 

 be formed. 



All these experiments give positive indications that the cyto- 

 plasm of the fertilised egg cannot contain a complex spatial 

 structure, in which each part represents one definite organ, 

 or part of the body, of the future embryo. For if this were true, 

 the division of such a structure into two halves (as in the 

 experiments with isolated blastomeres) would be bound to 

 cause defects in the embryos developing from each half. 

 Further, it is very difficult to imagine how fusion of two of 

 these systems could again result in a unity of entirely similar 

 properties. Finally, one cannot understand how such a struc- 

 ture, once it has been disturbed (as in the centrifuge experi- 

 ments), could return to its original state. 



The conclusion obviously must be that the egg cytoplasm 

 has no such complicated structure, no "extensive multiplicity", 

 but that it is a more or less homogeneous system. Such a system 

 — a water drop is a convenient model — can be divided, and, 

 alternately, two such systems can fuse, without any change 

 of properties. A change which produces heterogeneity will be 

 undone as soon as the factors which caused it suspend their 

 activity, because the components of the system return to their 

 position of equilibrium, i.e. the homogeneous distribution. 



