10 PRINCIPLES OF EMBRYOLOGY 



bility, that the ancestors of the whole phylum of vertebrates are to be 

 found in the larval forms of echinoderms. 



The type of analogical thinking which leads to theories that develop- 

 ment is based on the recapitulation of ancestral stages or the like no longer 

 seems at all convincing or even very interesting to biologists. Our interests 

 have been awakened by the possibility of an analysis of development in 

 causal terms ; and it is in this field that modern embryology seeks for its 

 guiding principles. Recapitulation, in all the forms in which it occurs, 

 remains an important phenomenon, but it appears nowadays as a series of 

 problems for evolutionary theory to discuss rather than as an explanation 

 of developmental processes. 



4. The mechanisms of development 



During the first phases in the study of a subject, all the available re- 

 sources have usually to be concentrated on the task of providing a 

 thorough scientific description of the phenomena involved. Embryology 

 remained in this condition until about the end of the nineteenth century, 

 when the first serious attempts were made to investigate the causal pro- 

 cesses by which developmental changes are brought about. The leader in 

 this endeavour was Wilhelm Roux, who coined the title 'Entwicklungs- 

 mechanik' for such studies. This word is still commonly employed in 

 German. Its hteral translation in English is 'developmental mechanics', a 

 phrase which is not only rather long and clumsy as the name of a branch 

 of science, but which carries a perhaps unfortunate suggestion that only 

 machine-like, physical processes are being envisaged. Another rather 

 awkward phrase, 'experimental embryology', is often used in EngHsh in 

 its place. Perhaps the most satisfactory expression would be 'epigenetics'. 

 This is derived from the Greek word epigenesis, which Aristotle used for 

 the theory that development is brought about through a series of causal 

 interactions between the various parts; it also reminds one that genetic 

 factors are among the most important determinants of development. It is, 

 however, not yet in common use. 



Since the beginning of this century, the experimental study of develop- 

 ment has been steadily growing in importance, and is now just as indis- 

 pensable a part of the science of embryology as is the purely descriptive 

 part. Before we proceed to the detailed discussion of different types of 

 embryos, it will be as well to give a general survey of the experimental 

 results in as broad an outline as the summary description of the successive 

 stages of development in the last section. 



The study of epigenetic processes has been carried out by two radically 

 different methods; those of experimental embryology proper, which 



