EXPERIMENTAL MORPHOGENESIS 151 



of our problems of morphogenesis and their solutions, will 

 be the best preparation for the philosophical part of these 

 lectures. 



HARMONIOUS-EQUIPOTENTIAL SYSTEMS FORMED BY 



WANDERING CELLS 



All of the harmonious-equipotential systems which we 

 have studied so far were the bases of histological 

 differentiation ; that is to say, the processes of their 

 differentiation consisted in specifically localised elements 

 of theirs becoming different in situ. Now we know at 

 least one type of systems which also may be called 

 harmonious-equipotential, but the differentiation of which 

 does not simply relate to elements at a fixed place. An 

 additional phenomenon enters here into the sphere of the 

 others. The elements not only become different where 

 they are, but a specific changing of locality, a specific kind 

 of wandering, goes hand-in-hand with differences relating 

 to the prospective value to be attained. I am speaking of 

 the formation of the larval skeleton of our well-known 

 Echinus. We know that the mesenchyme cells, which have 

 left the blastoderm and are arranged in a sort of ring of 

 bilateral structure, are the starting-point of this skeleton : 

 it indeed originates in a sort of secretive process on the 

 part of the cells ; the cells are moving about and are 

 secreting carbonate of lime during their wandering. The 

 experiments now have shown, as we know, that a whole, 

 though smaller, skeleton may also be formed, if only 

 a half or a quarter of the mesenchyme cells are present, as 

 happens to be the case in all experiments with isolated 



