THE STRUCTURE OF THE FERTILISED EGG 



29 



Fig, 10. Gastrula and pluteus of a sea urchin. Left: from a whole 

 egg; right: from a lo-blastomere. The latter are smaller, but har- 

 moniously built. After Morgan. 



that even blastomeres isolated at the 4-cell stage could still 

 develop into harmoniously built larvae of reduced size. 



The same phenomenon, the origin of normal embryos from 

 one half or one quarter of an egg, was found to occur in other 

 groups of animals as well. In several species of newts, for 

 example, the two blastomeres of the two-cell stage can be 

 separated by constricting the egg with a hair ligature in the 

 cleavage plane. Here, too, under certain circumstances both 

 halves will develop into harmoniously built embryos of half 

 the normal size, which lie together in one egg capsule (Spemann, 

 1903), (Plate I). Even if newt eggs are divided into two halves 

 at a much older stage, after the completion of cleavage, each 

 half may still develop into a complete, normal embryo. 



From other eggs, large parts of the egg cytoplasm can be 



