82 THE BIOLOGY OF THE FROG chap. 



Many of the older writers who speculated on the problem 

 of development were of the opinion that the egg contains in 

 miniature all of the organs of the body in the same form and 

 relation in which they occur in the adult, and that the parts 

 simply expand as nutriment is absorbed until the embryo 

 attains its maximum size. The process of development was 

 compared to the 'unfolding of a flower from a bud in which the 

 parts occur in the same relative position as in the expanded 

 blossom and simply unfold through the absorption of sap until 

 the flower reaches its final form. This view, which is known 

 as evolution ox preformation, was championed by such men 

 as Haller, Leibnitz, Bonnet, and Spallanzani ; it really 

 amounted to a denial of development. What appears to be 

 such, according to this interpretation, is simply growth, the 

 expansion of something preformed. The Abbe Spallanzani, 

 who made extensive and most excellent studies on the breed- 

 ing habits of Amphibians, considered the eggs of these ani- 

 mals to be small embryos of tadpoles whose parts through 

 the influence of the spermatic fluid were stimulated to growth 

 and activity. Other writers, among whom C. F. Wolff occu- 

 pies the most prominent place, regarded the ^gg as a mass 

 of simple unorganized material which becomes more and 

 more complex as development proceeds. This doctrine, 

 which is generally known as -the theory of epigenesis, has 

 become more widely accepted in recent times, although no 

 one at present espouses either epigensis or evolution in the 

 forms advocated in the eighteenth century. 



The cell theory, or the doctrine that animals and plants 

 are composed of living units, or cells, which was promulgated 

 by Schleiden and Schwann in 1839, did much to correct the 

 extravagant forms of the older theories of evolution, and 

 started investigators on the road to a truer conception of 

 development. A farther step in advance was made when 



