1 8 HOW ANIMALS DEVELOP 



has been found that the way any part develops is 

 controlled in such a way that all the material which 

 is available is worked up into one whole animal; 

 and further, it has been shown that this integrating 

 control is exerted by one particular part of the 

 embryo. At this early stage, then, the embryo is 

 very highly organized, because the way any part 

 behaves in development cannot be described without 

 referring to this special controlling part. The con- 

 trolling part is therefore called the Organizer. I shall 

 devote quite a considerable amount of space to a 

 consideration of the organizers which have already 

 been discovered, and the way in which they throw 

 light on the other facts which have emerged in the 

 study of development. 



The main interest of embryology at present is 

 theoretical, in the way discussed above. But there 

 are a very large number of important practical 

 questions which we may hope to be able to tackle 

 later, when the science has been worked out more 

 fully. For instance, why do most of the higher 

 animals, including man, lose the power of regenera- 

 tion so early in life, long before they are born? It 

 would be very convenient if we could regenerate an 

 amputated leg. Again, why do some cells start to 

 form an unorganized cancerous growth which the 

 animal cannot control, escaping from the agents 

 which keep the parts of an organism together as one 

 whole? How can we affect the production of twins 

 from one ^gg'^ The answers to these questions are 

 not, I think, right over the horizon of our present 



