THEORIES REGARDING ORGANIZATION 39 



Theories regarding organization 



This generalization brings us back again to a consideration of the theories 

 of early embryonic development. In general, there are two types of theories. 

 One is a theory of quantitative differences in the egg, involving differences 

 in the amount of some substance or substances. The other theory, of qualita- 

 tive differences in the egg, postulates different chemical compounds as being 

 responsible for the development of different structures. 



A complete and comprehensive theory of development has been based on 

 the idea that simple quantitative differences in metabolism may account for the 

 differences in the development of the parts of an egg. This concept is some- 

 times called a gradient theory. In general terms it states that the parts of the 

 egg develop according to their graded rates of metabolism. There is a further 

 corollary that the region of highest metabolism dominates or controls other 

 regions of the egg, producing unified, orderly development. This is a theory 

 of purely quantitative differences in the egg as a first cause of differences in 

 development. 



If two parts of an egg, A and B, differ by some quantitative factor, the 

 part A, which contains the greater quantity, will compete more successfully 

 for the substances necessary for development and may use up all of some 

 substance at the expense of B. B will not be able to develop into the same 

 structure as A, and thus competition may lead to different modes of develop- 

 ment. 



The other theory assumes that the parts of the egg are qualitatively 

 different — that is to say, they are different in chemical constitution and com- 

 posed of different substances. The region destined to become endoderm would 

 contain a chemical substance different from the other regions in the egg. 



Substances which are assumed to be present within a part and to cause 

 that part to develop in a specific way are called morphogenetic substances. 

 How do these various morphogenetic substances come to be localized in the 

 egg to form a pattern ? Early embryologists assumed that the substances were 

 at first mixed in the protoplasm of the egg and then became segregated into 

 definite regions. This concept throws the burden of the argument upon the 

 means by which morphogenetic substances segregate from one another into 

 localized regions of the egg. Various forces, such as electrical currents and 

 diffusion, have been suggested as the causes for the separation. 



