CHAPTER XII 



Some final considerations 



How does the spatial multiplicity of the adult organism arise 

 in the course of its development? This is the central problem, 

 underlying the whole of the present discussion. We have seen 

 that the spatial multiplicity as such is not yet present in the 

 fertilised egg. The latter shows only a slight extensive multi- 

 plicity, expressed in its polarity and bilateral symmetry, and 

 probably located mainly in the egg cortex. Development, how- 

 ever, involves a steady increase in the complexity of the 

 structure of the embryo. This begins with the local accumulation 

 of preformed determining substances under the influence of 

 directive activities originating in the primary coordinate system. 

 The parts now begin to influence each other. The next step 

 is the formation and local concentration of new substances in 

 cytoplasm and nucleus, leading to a rapid increase in the 

 chemodifferentiation of the egg. This, in turn, starts certain 

 topogenetic processes which result in migrations of cell material, 

 and thereby create new topographical relationships which set 

 the scene for new mutual influences, called induction. Finally, 

 the physical and chemical variety of the cells caused by their 

 chemodifferentiation becomes outwardly visible. This marks the 

 beginning of tissue differentiation, which prepares the organs 

 for their definitive functions in the organism. Even after the 

 attainment of the functional stage, development does not come 

 to a stop, but it goes on under continuous interaction of the 

 parts. Last of all, the phenomena of regeneration show that 

 even in the adult animal the parts still interact ceaselessly: 

 removal of a part of the body causes new processes in the 

 remaining part which result in complete or partial restitution 

 of the missing part. 



To a very great extent, therefore, development has the 



