THE PROBLEMS OF DEVELOPMENT 



SINGLE EMBRYO FORMS 



FUSION OF TWO EGGS 



Fig. 4. Action begins at left of diagram. Two fertilized amphibian eggs, 

 AB, CD, are placed side by side. Each egg then divides once, forming two two- 

 cell eggs. One two-cell egg, CD, is rotated through 90° and placed on top of the 

 other, AB, with the result that ABCD stick together. The fused eggs continue 

 to develop and form a single embryo. What sort of structure must the eggs AB 

 and CD possess in order that they may fuse and form only one embryo ? 



and, as a matter of fact, the parts of an egg can be interchanged without 

 any resultant abnormalities in development. For example, in an early 

 stage of development of the amphibian egg — it is important that it be an 

 early stage— it is possible to interchange pieces of the egg destined to be- 

 come skin with pieces destined to become the nervous system. The adult 

 skin is one type of structure; the adult nervous tissue quite another. But 

 they can be interchanged in an early stage. 



Figure 5 shows two eggs. A piece of the future skin is cut from one egg 

 and interchanged with a piece of the future brain of the other egg. Later 

 the part that was originally destined to be skin can be recognized as a part 

 of the primitive nervous system of the embryo which received it. In its new 

 position it forms a fairly normal part of the nervous system. And if it comes 

 from the same species it forms a perfect nervous system. In Figure 5 the 

 skin which became nervous tissue looks different because it came from a 

 different species. The use of two different species was intentional in order 

 to show very clearly the part which was originally transplanted. Thus, cells 

 normally destined to form skin can become nervous tissue, nerve fibers, and 



