THE ADDITION OF DETAILS 83 



Mosaic Eggs 



The stage of development which has just been 

 described, in which all the different parts of the 

 embryo arc determined, is called the mosaic stage, 

 because then each separate part is independent and 

 self-sufficient like the separate stones in a mosaic. In 

 most embryos this stage is attained gradually, as the 

 successive organization centres do their work. The 

 primary organization centre cuts the embryo up 

 into a mosaic made up of a few big blocks, the 

 neural plate and skin, and so on, then the secondary 

 organizers cut up those blocks into smaller ones, the 

 eyes, ears, legs, and other organs. But there are 

 some eggs, to which we have so far paid very little 

 attention, in which the mosaic stage is reached very 

 early in development, so that there is no time for us 

 to perform experiments to find out whether there are 

 any organization centres or not. In the most extreme 

 cases the egg is a mosaic immediately after it is 

 fertilized. In these eggs, of which the Ascidian, or 

 sea-squirt, egg is one of the best known examples, 

 we can sometimes show that there are definite sub- 

 stances each of which is essential for the formation 

 of some particular organ. They are known as organ- 

 forming substances. We do not know what they are, 

 but in the eggs of some species of animals they have 

 characteristic colours or textures so that they can 

 be easily recognized. The essential fact about them 

 is that if they are moved into an abnormal position 

 in the egg, the particular organ into which they 



