Cellular Differentiation 77 



cells and some of these are removed, the remaining cells form an incom- 

 plete embryo, with specific parts missing according to which of the cells 

 were removed. (This is called mosaic development.) In contrast, the em- 

 bryos of frogs, salamanders, and chicks are more malleable. Portions of 

 the early frog embryo can be removed without reducing its capacity to 

 turn out a normal product. The remaining cells simply take over the func- 

 tions of the missing ones. (This is called regulative development.) But 

 even here, if we wait long enough and permit the embryo to develop far 

 enough, removal will result in incomplete development. We might con- 

 clude from the above that, during development, the cells remaining after 

 excision had lost the capacity to replace the missing contingent. But we 

 can equally well imagine that by the time the excision is made, the en- 

 vironmental conditions no longer permit the transformation of a cell from 

 one type to another. 



In recent years an elegant set of experiments has shown that cell 

 nuclei taken from late embryonic stages are no longer equivalent to the 

 nucleus of the egg cell from which they were derived. Frog eggs were 

 enucleated; that is, the nucleus was removed with a microtool. It was re- 

 placed with a nucleus taken from a cell at the blastula or gastrula or 

 neurula stage of frog embryogenesis. In other words, the egg cytoplasm 

 was combined with a nucleus representative of cells that had already 

 become differentiated. This synthetic egg was then allowed to develop 

 in order to determine the degree of normality and completeness achieved. 



When a blastula nucleus was added to the enucleated egg, a normal 

 embryo resulted. When nuclei from later stages were employed, the re- 

 sulting embryos were grossly abnormal. Most important, the kind of ab- 

 normality observed depended on the part of the embryo from which the 

 nucleus had been taken. Thus, an endoderm nucleus added to the enu- 

 cleated egg gave rise to an embryo whose structures, derived from endo- 

 derm, were normal, but whose other structures were abnormal. A meso- 

 derm nucleus yielded an embryo whose mesodermal development was 

 normal, but whose other parts were abnormal. For reasons we have not 

 space to deal with here, these experiments and the ones mentioned earlier 

 do not conclusively demonstrate that a change takes place in the genetic 

 elements of cells undergoing differentiation, but they certainly do not 

 argue against this thesis. 



Conclusion 



We can now define cellular differentiation in a general way. We 

 know that the development of an organized multicellular structure always 

 involves the appearance of new cell types, different from their parents 



