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EMBRYOLOGY 



Fig. 53. An amphibian embryo 

 in a stage of transition from the 

 neurula to the functional larva. 

 Some structures still show the char- 

 acteristics of fields, while others are 

 differentiated in more detail. The 

 head (right) is better developed 

 than the rest of the body. Internally 

 the eye and brain are undergoing 

 structural differentiation. 



differentiation — that is, we assume that chemical differences exist between 

 the various fields. One field is chemically different from another, although 

 it is not visibly different. This is an invisible type of differentiation. Finally at 

 fa, when the fields actually begin to exhibit structure, we may speak of struc- 

 tural differentiation, which simply means a visible differentiation of the parts. 



There is still another way of looking at development. We may consider 

 it in terms of the potencies of tissues. It will be recalled that potencies are 

 simply abilities of the tissues to differentiate in various directions. Certainly 

 in the early egg there are many potencies. We have outlined them earlier: 

 When we get to the time fa, at which fields are present, the potencies become 

 more restricted and fewer in number. And when we come to the stage of 

 structural differentiation, we have reached the stage of having only single 

 potencies. Thus we find a progressive restriction in what the tissues can do. 

 Again a note of warning must be sounded. We pass beyond the experimental 

 facts when we maintain that the parts of the fully developed embryo possess 

 single potencies. Indeed the facts from studies on regeneration speak against 

 this rigid restriction of potencies. Cells in the adult may still possess latent 

 ability to differentiate into a new type of cell. 



Although the process of development has just been presented in three 

 steps, bear in mind what was said earlier about stages. Steps and stages are 

 purely arbitrary, and development is gradual and continuous. There is no 

 sharp time at which all structures become determined in all their character- 

 istics. The embryo in Figure 53 is in the process of transition from the 

 neurula, in which fields are determined, to the larva (Fig. 54), in which the 

 parts are determined within the fields. Thus in the embryo in Figure 53 the 

 eye already shows some structural differentiation, while the limb is merely 



