PROGRESSIVE DIFFERENTIATION WITHIN A FIELD 109 



Fig. 54. Development has reached the functional stage. The larva swims, 

 reacts to stimuli, possesses a circulation, takes in oxygen through the gills and 

 the skin, and begins to feed. Most of the parts show some structural differentia- 

 tion. The fields have given way to structures which exhibit self-differentiation 

 of their parts. Potencies are restricted and there is usually no replacement when 

 a developing structure is removed. 



represented by a field in the mesoderm. As will be seen in the next chapter, 

 some structures differentiate early in development whereas others appear 

 late in the process. In general, head structures develop earlier than those of 

 the remainder of the body. Note in Figure 54 the well-developed head and 

 forelimb but the absence of a hind-limb. 



Let us summarize these principles and attempt to see how they apply 

 as we follow the egg through the cleavage stages, through gastrulation and 

 the formation of the nervous system, and finally to the formation of ,the 

 individual structures in the embryo. 



First of all there is the principle of labile organization. This is an or- 

 ganization which may be changed by various kinds of experimental treat- 

 ment. In our study of the conversion of this labile organization into the actual 

 structures of the embryo, the principle of embryonic induction was revealed. 

 Some part of the egg contains factors which induce the remainder to differ- 

 entiate into various organ fields. Here the field concept arises. An entire 

 field will self-differentiate, but the parts of a field are not able to do so. 

 Thus the field exhibits a labile organization similar to that of an egg. Finally 

 the parts of the field become determined by embryonic induction. 



The over-all picture is one of a series of progressive changes so related 

 that each step in differentiation causes further differentiation until the process 

 is completed. This is something like a chain reaction, in which each reaction 

 sets off another. Therefore, in seeking the causes underlying the development 

 of any structure the student should examine carefully the preceding stages of 

 development for clues. 



In conclusion, it may be said that we have derived from the egg many of 



