64 HOW ANIMALS DEVELOP 



the purpose of making very minute operations on 

 embryos. They are not difficult to use moderately 

 well, but it requires years of practice to become 

 really skilful with them. For working on chick 

 embryos, which will be discussed on page 70 f, Spe- 

 mann's instruments are useless because the tissue is 

 too tough to be cut with the fine glass points; one 

 has to use coarser steel knives, which are rather more 

 difficult to handle. No one has yet invented instru- 

 ments really suitable for working on mammalian 

 embryos, and that is partly why we know so little 

 about them. The tissue is so sticky that it clings to 

 the end of a knife, and if one tries to scrape it off, it 

 usually gets torn to bits and spoilt; this small tech- 

 nical difficulty makes the most important experiments 

 impossible to perform. 



The Focus around which the Embryo is Integrated 



Speman's first experiment was to make an exchange 

 between a piece of white presumptive skin of 

 cristatus and a piece of brown presumptive neural 

 plate of taeniatus. He did the same operation at 

 various stages of development, and found that if the 

 exchange was made between two early gastrulae 

 the result was quite unlike what happened if it was 

 made between late gastrulae. When the experiment 

 was done in the early stage, the piece of white 

 cristatus presumptive skin which had been grafted 

 into the neural plate region of the taeniatus cgg^ 

 developed into neural plate like its surroundings; 

 and the brown taeniatus presumptive neural plate, 



