356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



ample, has postulated for the sex-genes. In the last ease the theory 

 is dealing with the development of hybrid embryos whose sex-genes 

 are assumed to have different rates of activity. 



A third view may also be permissible. Instead of all the genes 

 acting in the same way all the time or instead of certain kinds of 

 genes coming successively into action, we might postulate that the 

 kind of activity of all the genes is changed in response to the kind 

 of protoplasm in which they lie. This interpretation may seem less 

 forced than the others, and in better accord with the functional 

 activity of the adult organ-systems. 



We must wait until experiments can be devised that will help us 

 to discriminate between these several possibilities. In fact, geneti- 

 cists all over the world are today trying to find methods that will 

 help to determine the relation of genes to embryonic and adult char- 

 acters. The j)roblem (or problems) is being approached both from 

 a study of chemical changes that take place near the final steps in 

 organ formation, especially in the development of pigments, and 

 from a study of the early differentiation of the cell groups of the 

 embryo. 



We have also come to realize that the problem of development is 

 not as simple as I have so far assumed to be the case, for it depends, 

 not only on independent cell differentiation of individual cells, but 

 also on interactions between cells, both in the early stages of develop- 

 ment and on the action of hormones on the adult organ systems. 

 At the end of the last century, when experimental embryology 

 greatly flourished, some of the most thoughtful students of embryol- 

 ogy laid emphasis on the importance of the interaction of the parts 

 on each other, in contrast to the theories of Eoux and Weismann 

 that attempted to explain development as a progressive series of 

 events that are the outcome of self-differentiating processes or, as 

 we would say today, by the sorting out of genes during the cleavage 

 of the egg. At that time there was almost no experimental evidence 

 as to the nature of the postulated interaction of the cells. The idea 

 was a generalization rather than an experimentally determined con- 

 clusion, and, unfortunately, took a metaphysical turn. 



Today this has changed, and owing mainly to the extensive ex- 

 periments of the Spemann School of Germany, and to the brilliant 

 results of Horstadius, of Stockholm, we have positive evidence of 

 the far-reaching importance of interactions between the cells of dif- 

 ferent regions of the developing egg. This implies that original 

 differences are already present, either in the undivided egg or in the 

 early formed cells of different regions. From the point of view 

 under consideration results of this kind are of interest because they 

 bring up once more, in a slightly different form, the problem as to 



