202 PHYSIOLOGICAL GENETICS 



the newly diversified areas, and so on. Induction, then, is ;i 

 process that provokes pattern formation but docs not explain it. 



Self-differentiation means ili.it for the area in question pattern 

 formation has been ended. It will not be subdivided any more. 

 [f it occurs late in development, it will he the final outcome and 

 the end of the series of inductive processes. If it begins early in 

 development, it means that pattern formation has occurred 

 early. In these cases, it might have been preceded by induction, 

 but it might also be that induction had taken place so early in 

 development, even within the egg, that it would actually coincide 

 with pattern formation. This would mean that the inductor 

 is produced within the area upon which it acts as an evocator of 

 pattern. (Example: the induction of pattern in the egg as a 

 consequence of the bursting of the germinal vesicle.) Induction, 

 then, is one of the methods of initiating pattern formation. It 

 therefore does not explain pattern formation. This is an impor- 

 tant point, because the beauty of the experiments with the 

 "organizer" has led to a general belief that here the causative 

 principle of development was discovered. In fact, only an 

 evocator (Waddington) was found which in certain stages and in 

 certain cases acts like a fuse to set in motion the actual pattern- 

 forming process. 



The following processes, then, have been found by experi- 

 mental embryology to combine in development, according to 

 our notion: 



1. The ability of different embryonic areas to be broken up 

 into subareas according to a definite pattern; a process to be 

 followed later by a similar subdivision of these subareas. 



2. The possibility that these pattern formations may be started 

 at different times in development of different species, either in the 

 whole embryo or within different parts of the same embryo. 

 Late patterning means equipotential development; early pat- 

 terning, mosaic development. 



3. The breaking up of areas (or embryonic fields) into sub- 

 areas finally leads to a point where each area is so restricted in 

 potency that only one type of further differentiation is, as a rule, 

 possible. The area is determined. 



4. The breaking up of one area into subareas, the process of 

 patterning occurs in orderly fashion both as to time and as to 

 localization in normal development. Regardless of all other 



