62 HOW ANIMALS DEVELOP 



developmental fate under any conditions in which it 

 can develop at all, or, on the other hand, the 

 development of the piece might be directed by some- 

 thing outside it. Both possibilities are actually 

 realized. In the first case the piece is said to be 

 determined and to be capable of self- differentiation. In 

 the second case the tissue is undetermined, and the 

 external factor which determines it may be either 

 in the other parts of the tgg or else outside the tgg 

 altogether. It is unlikely that all the determiners will 

 be outside the tgg^ since it is difficult to suppose that, 

 if an Ggg is floating in the sea, for instance, the sea 

 water surrounding its different parts would be 

 sufficiently dissimilar to cause the development of a 

 whole set of organs. Actually the external determiners 

 are mainly important in very early development 

 though they are active again to cause minor modifi- 

 cations towards the end of embryonic life. The first 

 difference between the various parts of an tgg must 

 in any case be externally determined, either during 

 the process by which the egg is formed in the body 

 of the mother or soon afterwards; if the o^gg is 

 originally made with all its parts exactly alike it 

 cannot on its own develop special characteristics in 

 its different parts because there is no reason why a 

 special character should arise in one part rather than 

 another. Actually it is found that all eggs are made 

 with certain differences between the parts ; there is, 

 for instance, an animal pole at the top and a vege- 

 tative pole where the yolk is collected at the bottom. 

 Often the entry of the sperm at a definite point is a 



