172 THE PATH OF SCIENCE 



development was impossible. Then, in the early part of the 

 present century, Hans Spemann of Freiburg was able to local- 

 ize in early embryos the actual substances that "organize" its 

 further development. And W. Vogt of Munich, by marking 

 spots with stains on the surface of living embryos, has watched 

 and recorded the complex movements of cells during differ- 

 entiation. These men and others have made real progress in 

 investigating the causes that transform a simple egg into a 

 complex adult body, the old problem that Wolff started to 

 attack nearly two centuries ago. This surely should be a very 

 attractive problem for present-day physiologists, but it is only 

 one among many that await solution by a fully integrated 

 science of biology, in which animal physiology will take its 

 natural place. 



