38 HOW ANIMALS DEVELOP 



a whole embryo. In fact, the egg developed into 

 twins. This experiment proved that any cleavage 

 nucleus can develop into a whole embryo, and that 



Fig. 7. — Spemann's tying-up experiment. (From Spemann.) The 



upper half has cleaved several times and a nucleus has just passed 



down into the bottom half, which has cleaved once 



they are therefore all the same. Changes probably 

 go on in the nucleus in later stages of development ; 

 but they are not important in the cleavage period, 

 and do not determine how the parts of the embryo 

 will develop. Even in later stages of development 

 the genes still seem to be present in the nucleus and 



