38 FREE AND FIXED ANIMALS 



the free Appendicular^ (tailed, pelagic Ascidians 

 or Tunicata) are to be derived from fixed or from 

 free-swimming ancestors. These questions may- 

 be resolved into one, that of the statozoic origin 

 of Medusae and Appendicularise respectively. 

 Analogy affords no safe guide in solving such 

 a riddle as this. There must be some common 

 principle lying at the root of such a widely 

 spread phenomenon as that of the fixation of 

 animals ; and in my judgment the principle is 

 expressed in the term stereotropism invented 

 by J. Loeb. 1 



As a typical example of the positive working 

 of the stereotropic reaction Loeb quotes Dewitz's 

 account (1885- 1886) of the fertilisation of the 

 egg of the cockroach. The spermatozoa are 

 attracted by any surface ; when examined in a 

 drop of salt solution under the microscope they 

 are seen to be adhering to the slide and cover- 

 glass, none being free in the middle of the drop. 

 If a glass bead is introduced into the drop they 

 adhere to that, never leaving it. "When once 

 on the surface of the egg, the spermatozoa can 

 no more leave it, but must move on its surface 

 incessantly. In this way one spermatozoon finally 

 reaches the micropyle and gets into the egg. 

 The impregnation of the egg is therefore in 



1 Jacques Loeb, "Dynamics of Living Matter," New York, 1906, 

 p. 156. Stereotropism implies contact requirements or the reaction 

 to hard surfaces. 



