THE FINAL ADJUSTMENTS I33 



Summing-up 



This book started by considering the classical prob- 

 lems of embryology, with which biologists have been 

 concerned for centuries. We saw how the mere descrip- 

 tion of the patterns which are found led to the reca- 

 pitulation hypothesis and the discovery of the funda- 

 mental pattern of the three germ layers. We then saw 

 in outline how the elements of the pattern arise. 

 Processes go on in the egg which produce organ form- 

 ing substances, or evocators and competent tissues 

 which react to give determined organ rudiments. 

 These processes have two aspects. One is the "substance 

 aspect", which is concerned with the changes in the 

 nature of the substances by which the cells are com- 

 posed, which alters gradually as various regions of the 

 Ggg develop into cells of the nerves, muscles, kidneys, 

 liver, etc. The other is the "pattern aspect". Each cell 

 and organ of the body has a definite shape and struc- 

 tural organisation. The progress of understanding in 

 recent years has been in the main a digging down into 

 the basic processes underlying the substance aspect. 

 We have just seen in the last chapter how far we have 

 already got in understanding how hereditary genes 

 control the production of particular proteins. The 

 whole of the substance aspect of development can per- 

 haps be considered as an enormously elaborate and 

 complex composite made up out of elementary unit 

 processes of this kind. But a complete science can never 



