PROGRESSIVE DIFFERENTIATION WITHIN A FIELD 107 



Since determination is not final, some embryologists rightly object to the 

 use of this term. What has actually been demonstrated in the foregoing 

 experiments is a progressive change in the degree of self-differentiation of 

 structure with time. That this increase is necessarily accompanied by an 

 irreversible determination of the fate of a cell must not be assumed. There are 

 at least four ways of considering the changes which occur in the developing 

 egg with respect to time. The whole series of steps occurring during the 

 phase of development which we have just considered may be summarized in 

 tabular form. Let us indicate three different stages in time: h, h, and h — 

 t\ being the earliest of these stages and /•? the latest. Now consider what 

 happens with regard to what we call determination in the embryo. At a very 

 early stage, h, we can find little or no determination". It is very difficult to 

 speak of any determination in the early cleavage stages, since the egg is so 

 labile. However, there comes a stage at h when we can speak of the deter- 

 mination of organ fields, because fragments of each one of these fields placed 

 in a foreign environment totally apart from the rest of the embryo may 

 develop into perfect organs. As we have just pointed out, however, within 

 the organ field there is still little or no determination. The determination of 

 parts within the field takes place at some later stage, at tz. At this stage we 

 ■can demonstrate the determination of parts of the field. This way of looking 

 at development emphasizes a progressive determination of structure in the 

 embryo with time. 



Progressive Stages in Development 



Time Determination Differentiation Potencies Self-differentiation 



tj little or none none unlimited little or none 



t 2 organ fields chemical few major organs 



t 3 within the field structural one( ?) parts of organ 



We can also look at development in another way if we talk about the 

 degree of differentiation. In the early stage, h, we must classify the egg as 

 relatively undifferentiated. There is certainly no visible structural differentia- 

 tion in the early stages, and probably very little chemical differentiation. At 

 t-2, however, there is some sort of differentiation. The various fields which 

 exist at this time must be different from each other because they obviously 

 give such different results on transplantation. However, there is as yet no 

 structural differentiation in these fields. One cannot see differences, and there- 

 fore the differentiation that is assumed to exist is sometimes called a chemo- 



