174 READINGS IN BIOLOGICAL SCIENCE 



fluence of this theory, and today the embryology of all types of animals 

 is known, often in the most minute detail. In small transparent eggs the 

 developmental stages may be followed under the microscope; and, even 

 in eggs that are more opaque, technical methods have been devised that 

 reveal the changes taking place beneath the surface. The perfection of 

 these methods — staining, imbedding in paraffin, cutting into thin slices, 

 mounting these in balsam on glass slides, and reconstructing the whole in 

 wax — occupied for a long time the attention of a great number of profes- 

 sional embryologists, to the exclusion of considerations dealing with the 

 physical and chemical events that lie behind these visible stages of develop- 

 ment. The historical appeal was irresistible, especially if one believed that 

 what he was seeing and describing was the history of "creation" — or, as 

 it was called, evolution. There was soon established an immense body of 

 information concerning the development of all the main animal forms. 

 Accurate observation was called for, of the same order as that of all pic- 

 torial art. Beautiful illustrations of the development from egg to embryo 

 appeared in a host of monographs. The better the artist, the more brilliant 

 his performance. The anatomy of development became as well known as 

 the older anatomy of adult structures that had hkewise called for close 

 observation and an artistic sense of representation in color and perspec- 

 tive. 



During the final years of the last century and down to the present time a 

 new interest appeared, called experimental embryology, and sometimes 

 developmental mechanics. The reaction that had set in against the old 

 interpretation of the developmental stages as a recapitulation of the an- 

 cestry was in part responsible for this change of interest. New ways of 

 finding out something of what is going on behind the scene, the discovery 

 of potentialities in the egg never before suspected, the appHcation of 

 methods to bring about unnatural changes in the development, the emphasis 

 on the role of the environment in normal development, all conspired to 

 awaken new interest. 



Into the new fields of exploration many of the young embryologists 

 entered with renewed enthusiasm. A great deal was revealed and many 

 more problems, very different in kind from those' that had fascinated the 

 preceding generation, appeared. Here it seemed was the possibility of 

 further advance in an understanding of the developmental processes; and 

 the idea that embryology could be placed on an experimental basis was 

 especially attractive to those who were familiar with the great advances 

 that the experimental method in chemistry and physics had brought 

 about. The embryologist found himself dealing with problems so different 

 that it did not seem possible to apply at once the laws of chemistry and 

 physics. He dealt with such complex materials as proteins, colloids, and 

 with such complex problems as surface forces, permeability, etc., that the 

 physical scientists themselves had not yet brought into line with the rest 



