84 HOW ANIMALS DEVELOP 



develop will be found to appear in this abnormal 

 position : or if one such substance is removed from 

 the egg altogether, the organ will simply fail to turn 

 up at all. 



If such a mosaic egg is cut in two, each half can only 

 develop into half an embryo since it only has half 

 the right organ-forming substances. This is quite 

 different from the result of Spemann's experiment, 

 which was mentioned in Chapter 11, where it was 

 described how he made one egg produce two whole 

 embryos by constricting it in half with a hair. As a 

 matter of fact, he was lucky in this particular ex- 

 periment ; in other cases he sometimes only got one 

 embryo. Because, after all, the newt's egg has got 

 one "organ-forming substance" or something very 

 like one, namely, the organizer. Spemann found 

 that only those halves which contained the organizer 

 developed into embryos, and he got one or two 

 embryos, according as the organizer was included 

 whole in one half, or cut in two and divided between 

 the two halves. 



Different embryos can be arranged in a complete 

 series between those which, Hke the newt, are late at 

 arriving at the mosaic stage, and those which arrive 

 early. In the former we can often prove the presence 

 of an organization centre, but in the latter we cannot 

 discover whether there is one or not. Of course in 

 these mosaic eggs there may be an organizer which 

 acts so early that we cannot find it, and induces the 

 formation of the organ-forming substances. But it is 

 impossible to explain all organ-forming substances by 



