236 EMBRYOLOGY 



tained chemical substances T and B, come from exactly the same region. 

 Therefore they have the same chemical composition. All we have done is to 

 cut through the body, and presumably the chemical constituents on both 

 sides of the cut are similar. The structures derived at the two sides of the 

 cut surface are different. If we make the cut at any other level we still get 

 the same result. Therefore the qualitative interpretation of labile organization 

 (Chap. 3) does not work out very well as applied to these particular tissues, 

 and we turn our attention to the quantitative interpretation. 



The quantitative theory simply assumes that a substance is present in high 

 concentration in the end which forms the tentacles, and that the concentration 

 decreases in such a way that it is lowest at the base (Fig. 159). Then when 

 the body is cut into two parts, in each of the parts the following relationship 

 obtains: a substance, S, is present in greater concentration at the end which 

 forms the tentacles. If we make the cut through the level S, M , each side of 

 this cut will have that concentration, S n .\. However, the concentration of 5 

 in the upper end of piece number 1 (S„) is higher than S„a and the tentacles 

 develop at this upper end while the base develops at S n _i . In piece number 2 

 the concentration of the upper end is S„a , which is higher than S n _ 2 , the con- 

 centration at the lower end, and therefore in piece number 2 the tentacles 

 develop at S n .i, the base at S n .2. In both cases the tentacles form at the region 

 of highest concentration of S. 



The quantitative theory seems to be more reasonable than the qualitative 

 theory. As we have already noted, the difficulty in assuming that different 

 substances are present at the same region of the body makes the qualitative 

 theory improbable. 



Origin of the cells for regeneration 



The process of regeneration gives rise to some new problems in develop- 

 ment. One of them concerns the origin of the cells which give rise to the 

 regenerating structure. We might visualize either that the new structure 

 comes from the old cells by migration or cell division or that there may be 

 reserve cells, embryonic in nature, which are undifferentiated and which give 

 rise to the new structure. 



During the regeneration of such a structure as the amphibian limb we 

 can follow the behavior of some of the cells. When a limb is severed several 

 types of tissue are involved in the cut. First there is the epidermis; then the 



