28 THE STRUCTURE OF THE FERTILISED EGG 



egg. As a consequence of the structure of the egg, in which 

 each part had its fixed place, the various organs, though each 

 developing independently, would later fit together as parts of 

 a "mosaic". Therefore, development would not involve an "in- 

 crease of spatial multiplicity", but only the manifestation of 

 a previously invisible, preformed, spatial multiplicity. This is 

 the hypothesis of neo-yre formation. Its alternative, that of 

 neo-epigenesis, is the hypothesis that the spatial multiplicity 

 of the embryo is in no way preformed in the egg, but that it 

 arises during development. 



W. Roux, the creator of the science of "developmental me- 

 chanics", considered the solution of this problem its first and 

 most important task. Since then, it has been the motive for a 

 great number of investigations in a wide variety of animal 

 groups. Such studies are characteristic of the first period of 

 research in developmental physiology. On the strength of these 

 results, we can now assert on good grounds that the spatial 

 structure of the egg is generally very simple, and that there- 

 fore the spatial multiplicity of the embryo increases strongly 

 during development, as is postulated by the neo-epigenetic 

 hypothesis. We shall now discuss a number of experiments 

 which have led to this conclusion. 



We have already seen that the first step in development is 

 the cleavage of the fertilised egg into two cells. Each of these 

 cells divides into two again, and so on, until a great number of 

 cleavage cells, or blastomeres, has been formed. This process 

 does not involve cell growth, so that the resulting blastula is 

 still of about the same size as the original egg. In 1891, Driesch 

 made an experiment which was to prove of the greatest im- 

 portance for the development of our views in this field. He took 

 sea urchin eggs which had just begun developing, and were 

 in the two-cell stage. Driesch shook those eggs in a test tube 

 with sea water, and thereby separated the two blastomeres of one 

 egg in a number of cases. He then studied the further devel- 

 opment of these isolated halves, and observed that each of 

 them was able to develop into a harmoniously built young larva. 

 These larvae were only half the size of ordinary ones, but were 

 otherwise normal in form (Fig. 10). Moreover, Driesch found 



