272 AN INTRODUCTION TO ZOOLOGY 



this theory grew the idea of encasement, i.e. the preformed em- 

 bryo must contain a second smaller miniature, and this another 

 ad infinitum, just as a series of boxes may be made to fit within 

 one another. Wolff did more than any other embryologist to 

 overthrow the preformation theory and to introduce in its stead 

 the idea of epigenesis. By epigenesis Wolff meant the gradual 

 formation of organs from an unorganized egg.- The establish- 

 ment of this theory changed all subsequent work in embryology. 

 Karl E. von Baer (1792-1876, Fig. 154) was the greatest of all 

 embryologists. His most important services were (i) the excel- 

 lence of his work, which raised the standard of embryological 

 investigations, (2) the establishment of the germ-layer theory, 

 and (3) the broadening of the field of embryology by making it 

 comparative. He is now known as the " father of modern em- 

 bryology." Francis M. Balfour (1851-1882) in 1880-1881 pub- 

 lished his Comparative Embryology. This work is a summary of 

 the enormous literature on the subject that had accumulated 

 during the period inaugurated by von Baer. It also contains the 

 result of much original research and many broad generalizations. 



e. Physiology 



The medical men of ancient times depended largely upon the 

 investigations of Galen. Diseases were supposed to be due to 

 the presence of spirits and humors in the body. This idea, called 

 the pneuma-theory, was not overthrown until the Revival of 

 Learning. The names of William Harvey (1578-1667), Albrecht 

 Haller (1708-1777), and Johannes Miiller (1801-1858) are among 

 the most famous in the field of physiology. Harvey demon- 

 strated the connections between arteries and veins, and discov- 

 ered the circulation of the blood. Although these contributions 

 to knowledge have made his name famous, the introduction by 

 him of experimental methods in physiological investigations have 

 had a more profound influence upon the progress of physiology. 



Haller separated the study of physiology from medicine and 



