152 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



blastomeres of the two or four-cell stage of cleavage. It 

 is clear that in these cases the performance of each single 

 cell must be different from what it is in the normal case, and 

 that the same sort of differences in the morphogenetic 

 performances appears again, if the two- and the four-cell 

 stage are compared with each other. And there are still 

 some other phenomena showing the possibility of different 

 performances being carried out by the individual cells. 

 Peter has shown that the number of mesenchyme cells 

 may vary enormously under certain conditions ; but, in 

 spite of that, the skeleton always will be complete. It 

 may be said that this line of research is only of a relative 

 value to our own questions, as, of course, variability relates 

 to different individuals : but it seems to me that it adds a 

 very good supplementary instance to what the experiment 

 on the individual itself has established. 



We should only be repeating ourselves if we were to 

 analyse again what happens here as the expression of 

 the harmonious-equipotentiality itself. But indeed there 

 occurs something new in this instance : the single mesen- 

 chyme cell not only has to perform in each case that single 

 act of specific secretion which the case requires, but it also 

 has to wander to the right place in order to perform it ; 

 there must be some order, not only about the acts of 

 secretion after wandering, but also in the migrations them- 

 selves. If undisturbed ontogeny alone were possible, and 

 if therefore a theory like that of Weismann were in place, 

 we might say perhaps that each mesenchyme-cell is specified 

 not only as to its performance in secretion, but also with 

 regard to its chemotactical irritability, the latter being 

 typically localised, so that its effect becomes typical, thanks 



