106 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



raorphogenetic influence of the nervous system upon processes 

 of restoration is more than indirect ; the movements of the 

 animal, which become very much reduced by the extirpation 

 of the ganglia, being one of the main conditions of a good 

 regeneration. 



Of course, all we have said about the importance of 

 special materials in the ripe germ, as bearing on specifically 

 localised organisations, might be discussed again in our 

 present chapter, and our intimate polar-bilateral structure 

 of germs may also be regarded as embracing formative 

 stimuli, at any rate as far as the actual poles of this 

 structure are concerned. This again would bring us to the 

 problem of so-called "polarity' in general, and to the 

 " inversion ' of polarity, that is to a phenomenon well 

 known in plants and in many hydroids and worms, viz., 

 that morphogenetic processes, especially of the type of 

 restitutions, occur differently, according as their point of 

 origin represents, so to speak, the positive or the negative, 

 the terminal or the basal end of an axis, but that under 

 certain conditions the reverse may also be the case. But a 

 fuller discussion of these important facts would lead us 

 deeper and deeper into the science of morphogenesis proper, 

 without being of much use for our future considerations. 



And so we may close this section l on formative stimuli 



1 A full analysis of the subject would not only have to deal with formative 

 stimuli as inaugurating morphogenetic processes, but also with those stimuli 

 which terminate or stop the single acts of morphogenesis. But little is 

 actually known about this topic, and therefore the reader must refer to my 

 other publications. I will only say here, that the end of each single morpho- 

 genetic act may either be determined at the very beginning or occur as an 

 actual stopping of a process which otherwise would go on for ever and ever ; 

 in the first case some terminating factors are included in the very nature of 

 the morphogenetic act itself. 



