86 HOW ANIMALS DEVELOP 



the active form which is found in the adult tissues. 

 The preparation of the organizer therefore involves 

 the activation of a substance. When we know more 

 about the exact chemical formula of the evocator, 

 we shall perhaps be able to find out how this 

 activation happens. 



The localization of the organizer, i.e. the deter- 

 mination of whereabouts in the egg the activation 

 shall happen, must be done by factors outside the 

 egg, partly by the way the egg is formed in the 

 ovary and, in the frog, at any rate, partly by the 

 place where the sperm enters the egg in fertilization. 

 The localization of organ-forming substances in 

 Ascidians and other mosaic eggs must also be done 

 by external agents, and here again the same two 

 factors are probably important. 



Human eggs are not mosaic. They probably 

 develop a primary organization centre which acts at 

 the stage corresponding to that in which the second 

 chick organizer is active, that is, at the primitive 

 streak stage. The embryo reaches this stage about 

 8 days after conception. Before then, separate 

 bits which may get split off by chance can develop 

 into whole embryos if they are big enough. The 

 most common sort of splitting is for the embryo to 

 fall into two halves: probably after the egg has 

 cleaved the first time the two daughter cells fail to 

 stick together for some reason. We then get identical 

 twins, which are not the same a,s fraternal twins, which 

 arise when two eggs are fertilized simultaneously. 

 Identical twins must, of course, have exactly the 



