98 POPULAR SCIENCE MONTHLY. 



But, strangely enough, the course of development in any higher 

 organism is not straightforward, but devious. The bird, as well as 

 all higher animals, acquires gill-clefts and other rudimentary structures 

 not adapted to its condition of life. Most of the rudimentary organs 

 are transitory and bear testimony, as hereditary survivals, to the line 

 of ancestry. They are clues by means of which phases in the evolu- 

 tion of animal life may be deciphered. 



Bearing in mind these shifting changes, one begins to see why the 

 adult structures of animals are so difficult to understand. They are 

 not only complex they are also greatly modified. The adult condi- 

 tion of any organ or tissue represents the last step in a series of 

 gradually acquired, modifications, and is, therefore, the farthest de- 

 parture from that which is ancestral and archetypal. But in the 

 process of formation all the simpler conditions are exhibited. If, 

 therefore, we wish to understand an organ or an animal we must fol- 

 low its development, and see it in simpler conditions, before the great 

 modifications have been added. 



The tracing of the stages whereby cells merge into tissues, tissues 

 into organs, and how the organs by combinations build up the body, is 

 embryology. It has become one of the richest and most suggestive of 

 the biological sciences in furnishing clues to the past history of 

 animals and throwing light on their relationships. 



It is the purpose of this paper to trace in a summary way the rise 

 of that interesting division of biological science, pointing out its 

 epochs, and telling something about the men who laid its foundations, 

 the discussion being limited to the animal side, with no attempt to 

 represent the rise of plant embryology. That we can ' read all his- 

 tory in the lives of a few great men ' is essentially true in reference 

 to the progress of embryology. There are many individual workers, 

 each contributing his share, but the ideas of the science are molded 

 into effective form only in the minds of the leaders. In this group 

 of ' the leaders,' Von Baer stands as a monumental figure, at the 

 parting of the ways between the new and the old — the sane thinker, 

 the great observer. 



The story of the rise of embryology can, for convenience, be di- 

 vided into five periods each marked by an advance in general knowl- 

 edge. These are: (1) the period of Harvey and Malpighi; (2) the 

 period of Wolff; (3) the period of Von Baer; (4) the period from Von 

 Baer to Balfour; and (5) the period of Balfour with an indication 

 of present tendencies. 



The Period of Harvey and Malpighi. 

 In General. — The conventional way of looking at the rise of em- 

 bryology has been derived mainly through the channels of German 

 scholarship. But there is reason to depart from the traditional aspect 



