THE LATER STAGES OF DEVELOPMENT 



153 







Fig. 54. Extirpation of the fore-limb primordium of Amblystoma 

 at an early stage (left), leads to the production of a larva (right), 

 in which the limb concerned is completely absent. After Harrison. 



onto the egg membranes of a chick embryo, they, too, were 

 found to continue their differentiation. 



The case of the mesonephros primordium of the chick embryo 

 deserves special mention. In normal development, the meso- 

 nephros of the chick becomes reduced from the tenth day of 

 incubation onwards, its function being taken over by the meta- 

 nephros. If grafted onto the egg membranes of an older embryo, 

 the primordium first differentiates into a typical mesonephros. 

 Reduction, however, sets in at about the sam.e time at which 

 it would have happened in the normal embryo (Danchakoff, 

 1924). This shows that not only the nature of the progressive 

 differentiation, but its duration as well is determined in the 

 organ primordium. Holtfreter (1931) found a similar case in 

 amphibians. Here, explanted endoderm of the prospective mid- 

 gut remains compact for several days, but it becomes broken 

 up into loose cells at the time at which, in the normal embryo, 

 the yolk-rich endoderm cells would have been expelled into the 

 mid-gut lumen. 



Apart from transplantations, explantations have also been 

 applied to the study of the properties of organ primordia. This 

 method proved that in many cases the primordia are capable 

 of independent differentiation outside the body, provided they 

 are cultured in a suitable medium. The heart rudiment of 

 amphibians, for instance, developed in vitro into a pulsating 

 heart in which the various parts of a normal heart were clearly 



