466 



ANIMAL BIOLOGY 



reducible — gave content to the term biology and created the sci- 

 ence of life in its modern form. (Fig. 9.) 



5. Embryology 



The cell theory resulted, as we have seen, from combined studies 

 on the adult structure and on the development of plants and ani- 

 mals, and accordingly implies that the science of embryology has a 

 history of its own. As a matter of fact, Aristotle discussed the 

 wonder of the beating heart in the hen's egg after three days' in- 

 cubation, but there the subject practically rested until Fabricius 



Fig. 303. — Theodor Schwann. 



(1537-1619) at Padua, early in the seventeenth century, published 

 a treatise which illustrated the obvious sequence of events within 

 the hen's egg to the time of hatching. This beginning was built 

 upon by a pupil of Fabricius, the celebrated Harvey, who added 

 many details of interest, though little progress in embryology was 

 possible without the microscope. 



The microscope was first turned on embryological problems by 

 the versatile Malpighi in two treatises published in 1672, and 

 at one step animal development was placed upon a plane so ad- 

 vanced that for over a century it was unappreciated. One conclu- 



