EXPERIMENTAL MORPHOGENESIS 77 



future actual course of this development, whether it be 

 undisturbed or disturbed in any way ; it is, so to say, the 

 actual, the real fate of our element, that we take in account. 

 I have proposed to call this real fate of each embryonic 

 part in this very definite line of morphogenesis its pro- 

 spective value (" prospective Bedeutung ' : in German). The 

 fundamental question of the first chapter of our analytical 

 theory of development may now be stated as follows : Is the 

 prospective value of each part of any state of the morpho- 

 genetic line constant, i.e. is it unchangeable, can it be nothing 

 but one ; or is it variable, may it change according to 

 different circumstances ? 



We first introduce a second concept : the term prospective 

 potency (" prospective Potenz " in German) of each embryonic 

 element. The term " prospective morphogenetic potency " is 

 to signify the possible fate of each of those elements. With 

 the aid of our two artificial concepts we are now able to 

 formulate our introductory question thus : Is the prospective 

 potency of each embryonic part fully given by its prospective 

 value in a certain definite case ; is it, so to say, identical 

 with it, or does the prospective potency contain more than 

 the prospective value of an element in a certain case reveals ? 



We know already from our historical sketch that the 

 latter is true : that the actual fate of a part need not be 

 identical with its possible fate, at least in many cases ; that 

 the potency of the first four blastomeres of the egg of the 

 sea-urchin, for instance, has a far wider range than is shown 

 by what each of them actually performs in even this 

 ontogeny. There are more morphogenetic possibilities con- 

 tained in each embryonic part than are actually realised in 

 a special morphogenetic case. 



