ONTOGENY 21 



as to furnish a basis of this kind sufficient for immediate 

 necessity. Next comes the study of the real processes leading 

 to the production of one condition out of another, processes 

 which underlie the externally visible form changes. This 

 physiological aspect of Embryology is concerned more with 

 how development occurs, how, and through the operation of 

 what factors or mechanisms, one condition leads to another. 

 In a way this is also the why of development not "why" in 

 the philosophical sense of course, but in the sense of "how 

 does it happen that" these things occur in development. 

 Here the two methods of observation and experiment are com- 

 bined and by the artificial modification or the elimination of 

 one condition of development after another, the essential 

 factors are discovered and their modes of operation determined. 

 The science of Embryology has now fairly entered upon this 

 stage and the dominant note of the subject to-day is this 

 search for underlying processes and modes of action. But as 

 yet it is impossible to say that we have reached the final 

 period of the formulation of the broad fundamental generali- 

 zations which give unity to the infinitely diverse phenomena of 

 development, and which are expressed in the form of laws. 

 While something has been accomplished in this direction, the 

 basis of fact is not as yet sufficiently broad, and the necessity 

 of frequent restatement of such "laws" shows their formulations 

 to be premature, save as guides in investigation. 



These steps in the development of the science of Embryology 

 do not so nearly represent the course of thought and hypothesis 

 as that of actual knowledge and achievement. For even in 

 the eighteenth century the earliest embryologists had their 

 hypotheses as to the causes of this mysterious process of develop- 

 ment. They offered first what seemed to them an explanation 

 of the facts of development which came to be termed the idea 

 of "evolution" or preformation. This idea was that within 

 the germ, either in the egg ("ovists") or in the spermatozoon 

 ("spermists, " "animalculists") there was contained a miniature 

 organism resembling, in a general, or even in a precise way, 

 the adult form (Fig. 12). And this miniature had merely to 



