THE GENESIS OF LIVING ORGANISMS 211 



the same-sized cells but only half the number. It follows that, 

 on the one hand, the number of cells is normally fixed by a 

 set number of division-impulses, and that, on the other hand, 

 the shaping impulses which come in later are to a great extent 

 independent of the quantity of material furnished them by the 

 division. 



The first divisions give a number of similar cells, and these 

 form a sphere which, by taking up water, becomes hollow. 

 As soon as this sphere has invaginated and become a hollow 

 two-walled sphere, remarkable changes appear in the cells. 

 Those in the outer wall, the so-called outer germ-layer, which 

 furnish the organs of the skin and the nervous system, lose 

 the power to form whole animals ; but each retains the power 

 to form the organs that come from the outer germ-layer. In 

 the same way, the cells of the inner wall, or inner germ-layer, 

 which furnish the digestive system, lose the power of forming 

 the organs of the outer layer. This also holds good for the 

 cells of the layer in between, the middle germ-layer, which 

 has meanwhile been split off from the others, a layer which 

 has the duty of forming the muscles and the bones. 



As soon as a germ-layer has been laid down, its cells each 

 retain only a limited formative power. It is natural to sup- 

 pose that, in the nuclei of all the cells of each of the three 

 germ-layers, only one-third of the genes are present, while 

 two-thirds disappear by unequal division of the chromatin 

 or are dissolved (genolysis). 



The same thing appears later on when the germinal areas 

 are mapped out within the germinal layers, areas which 

 are to produce certain organs. As soon as a germinal area is 

 marked off, the cells that compose it forego the power to form 

 the organs of another germinal area. 



It may be taken as a fundamental law of genesis that, 

 when a structure appears, this excludes the power to form 

 anything else. And so, as development proceeds, it is accom- 



