EXPERIMENTAL MORPHOGENESIS 99 



salts of lithium effect radical changes in development. 1 I 

 cannot describe fully here how the so-called " lithium 

 larva ' originates ; let me only mention that its endo- 

 derm is formed outside instead of inside, that it is far 

 too large, that there is a spherical mass between the 

 ectodermal and the endodermal part of the germ, that 

 a radial symmetry is established in place of the normal 

 bilateralism, that no skeleton exists, and that the mesenchyme 

 cells are placed in a quite abnormal position. All these 

 features, though abnormal, are typical of the development 

 in lithium. The larvae present no really pathological 

 appearance at all, and, therefore, it may indeed be said that 

 lithium salts are able to change fundamentally the whole 

 course of morphogenesis. It detracts nothing from the 

 importance of these discoveries that, at present, they stand 

 quite isolated : only with lithium salts has Herbst obtained 

 such strange results, and only upon the eggs of echinids, 

 not even upon those of asterids, do lithium salts act in this 

 way. 



7. THE FORMATIVE CAUSES OR STIMULI 



The Definition of Cause 



We cannot begin the study of the " causes " of the 

 differentiation of form without a few words of explanation 

 about the terminology which we shall apply. Causality 

 is the most disputed of all categories ; many modern 

 scientists, particularly in physics, try to avoid the concept 

 of cause altogether, and to replace it by mere functional 

 dependence in the mathematical meaning of the term. 



1 Zeitschr. wiss. Zool. 55, 1902 ; and Mitt. Neapel. 11, 1903. 



