82 HOW ANIMALS DEVELOP 



From this a well-formed leg had developed, and 

 the cells had turned into cartilage cells and were 

 beginning to turn into bone by the time the experi- 

 ment ended. The ends of the bones at the joints 

 of the hip and knee had nearly the right shape; 

 as we shall see later (p. 1 1 1), the actual functioning 

 of the joints and the movements which take place 

 there have a certain influence on the shape of 

 the articulating ends of the bones, but even without 

 this help the rudiment can develop quite a good joint- 

 surface. Further, not only did the bone-rudiment 

 develop its right shape and right sort of cells, but the 

 various chemical processes which go on in developing 

 bone also occurred. There is a particular substance 

 called phosphatase which appears at a certain stage 

 or normal development and is concerned with the lay- 

 ing down of the calcium phosphate out of which the 

 bone is made ; and it was shown that the isolated rudi- 

 ment had produced this substance at the right time. 

 Another very remarkable instance of the self- 

 sufficiency of organ rudiments after they have 

 become determined is found in the embryonic kidney 

 or mesonephros of the chicken. This structure only 

 exists in the embryo and degenerates and falls to 

 bits before the chick is born. If the rudiment is cut 

 out and isolated it goes on developing quite normally 

 for the right length of time and then punctually starts 

 to degenerate. A similar capacity for independent 

 development has been found in the organ-rudiments 

 of frogs and newts and most other animals which 

 have been investigated. 



