PROBLEM OF DIFFERENTIATION 375 



which result in the formation of the duct system and the acini are examples 

 of morphological differentiation. On the other hand, the invisible, subtle 

 change or changes which originally altered the respective cells of the nipple 

 area and, thereby, ordained or determined that the cells in this particular 

 locale should produce duct and nipple tissue is an example of biochemical 

 differentiation or chemodifferentiation. Chemodiflterentiation, morphological 

 differentiation, and physiological differentiation, therefore, represent the three 

 types or levels of differentiation. Moreover, all of these differentiations stem 

 from a persistent change in the fundamental activities of cells or cell parts. 



It should be observed further that chemodifferentiation represents the initial 

 step in the entire differentiation process, for it is this change which determines 

 or restricts the future possible activities and changes which the cell or cells 

 in a particular area may experience. Also, in many cases, differentiation ap- 

 pears to arise as a result of stimuli which are applied to the cell or cells exter- 

 nally. That is, internal changes within a cell may be called forth by an 

 environmental change applied to the cell from without. 



In embryological thinking, therefore, the word differentiation implies a 

 process of becoming something new and different from an antecedent, less- 

 differentiated condition. But beyond this, differentiation also connotes a cer- 

 tain suitableness or purposefulness of the structure which is differentiated. 

 Such a connotation, however, applies only to normal embryonic differentia- 

 tion; abnormal growths and monstrosities of many kinds may fulfill the first 

 phase (i.e., of producing something new) of differentiation as defined in 

 the first sentence of this paragraph, but they do not satisfy the criteria of 

 purpose and of suitableness within the organized economy of the developing 

 body as a whole. It is important to keep the latter implications in mind, for 

 various structures may appear to be vestigial or aberrant during embryonic 

 development, nevertheless their presence may assume an important, purpose- 

 ful status in the ultimate scheme which constructs the organization of the 

 developing body. 



2. Self-differentiation and Dependent Differentiation 



In the amphibian, very late blastula and beginning gastrula, the presumptive, 

 chordamesodermal area, when undisturbed and in its normal position in the 

 embryo, eventually differentiates into notochordal and mesodermal tissues. 

 This is true also when it is transplanted to other positions. That is, at this 

 period in the history of the chordamesodermal cells the ability resides within 

 the cells to differentiate into notochordal and mesodermal structures. Con- 

 sequently, these cells are not dependent upon surrounding or external factors 

 to induce or call forth differentiation in these specific directions. Embryonic 

 cells in this condition are described as self-differentiating (Roux). Similarly, 

 the entodermal area with its potential subareas of liver, foregut, and intestine 

 develops by itself and this area does not rely upon stimuli from other con- 



