GRADIENTS, FIELDS, AND DETERMINATION 299 



in part responsible for the character of much of developmental theory. 

 But the primary developmental pattern is no longer present in most ani- 

 mal eggs at the beginning of embryonic development; more or less struc- 

 tural organization is usually already present. Many buds and reconstitu- 

 tions of individuals from isolated pieces or cell aggregates involve origin 

 of a new developmental pattern, which is accidental as regards both time 

 and place, so far as the original individual is concerned. That these forms 

 of development are not merely the further expressions of organizations 

 already present in consequence of the particular past history of a single 

 cell, as embryonic development is, appears evident. How completely in- 

 dependent of pre-existing organization they may be will appear more 

 clearly in later chapters. It is to these forms of development, rather than 

 to the egg and embryo, that we must look for the beginnings of develop- 

 mental pattern and the physiological bases of determination and differ- 

 entiation of parts. 



DEDIFEERENTIATION 



Diflferences of opinion concerning the possibility of dedifferentiation 

 are, of course, associated with the question whether certain cells are or 

 are not differentiated. If cells which take part in the reconstitutional de- 

 velopment of other organs than those of which they were originally parts 

 are differentiated, they presumably undergo more or less dedifferentia- 

 tion. Those who regard them as differentiated usually conclude that they 

 dedifferentiate and redifferentiate in reaction to the altered environment. 

 Others apparently conclude that, because they reconstitute other organs, 

 they must be undifferentiated. The question of reversibihty or regressi- 

 bility of development in general is, of course, essentially the same ques- 

 tion and open to this difference of opinion. 



Weismannian theory necessarily assumed that reversal or regression of 

 differentiation is impossible, that a cell which has once begun to differen- 

 tiate can never return. Only the germ plasm does not differentiate. As a 

 matter of fact, however, the egg and the spermatozoon of most animals 

 appear to be very highly differentiated cells, perhaps the most highly 

 differentiated cells of the individual. They appear to have approached 

 or attained the limit of possible differentiation in two different directions. 

 The yolk is both a chemical and a structural differentiation; the motor 

 apparatus of spermatozoa can scarcely be regarded as undifferentiated 

 cytoplasm; and the sperm nucleus is certainly in very different condition 

 from an embryonic nucleus. When fully differentiated as gametes, both 



