THE "organization CENTRE" 63 



further important external determining factor. But 

 the number of these early external determiners is 

 comparatively small, and the most striking steps in 

 development are caused by internal determiners. 



The Technique of Aiicro-Surgery 



The first experiments which were successful in 

 locating these determiners inside the embryo were- 

 carried out by Spemann. He used two species of 

 newts, one of which, Triton cristatus, lays a white egg, 

 while the other, Triton taeniatus, lays a rather dark- 

 coloured egg. Spemann first had to work out a 

 technique for making operations on these eggs. The 

 eggs themselves are embedded in two layers of jelly, 

 but these can be removed fairly easily. However, 

 when the eggs have been made free in this way, 

 there are further difficulties because the eggs are so 

 small, only about 2 mm., or a tenth of an inch, 

 across. Spemann invented two special instruments 

 for cutting such small masses of tissue ; one is made 

 of very fine glass drawn out to a sharp point, much 

 finer than any point which can be made on a pin, and 

 the other is a little loop of baby's hair, mounted on a 

 holder, which can be used as a knife. All the opera- 

 tions have, of course, to be performed under a 

 microscope. 



The advance of experimental embryology is always 

 waiting on two things, the invention of new instru- 

 ments and methods, and the training of people with 

 sufficient manipulative skill to use them. Spemann's 

 instruments were the first which were invented for 



