70 HOW ANIMALS DEVELOP 



Birds 



This discovery is so fundamental that we must go 

 on to see if it applies to other animals and try to 

 find out if they also have organization centres. Work 

 which has been carried out since Spemann's original 

 experiments has shown that in fact organization 

 centres are not merely a speciality of the newt's tg^, 

 of no general importance, but, on the other hand, 

 can be found in nearly all groups of vertebrates and 

 in some invertebrates. The one which is perhaps 

 most similar to the amphibian organization centre 

 has been found in the birds, but, as we might expect, 

 there are differences correlated with the differences 

 in gastrulation in the two groups. 



As was described in the last chapter, the endoderm 

 and mesoderm are formed separately in the bird, 

 while in the newt they are both formed at the same 

 time as the blastopore. We find that not only are 

 the formative functions of the newt's blastopore 

 shared between two structures in the bird but so are 

 its organizing functions. There are, in fact, two bird 

 organizers instead of one ; the first is concerned with 

 the formation of the endoderm, the second with that 

 of the mesoderm. 



All the experiments with birds were done on 

 chicken or duck embryos which were kept in culture 

 after they had been operated on. The first experi- 

 ment was concerned with the endoderm. This is 

 already formed when the tgg is laid, so there is 

 no possibility of transplanting the ''endoderm- 



