GRADIENTS, FIELDS, AND DETERMINATION 295 



velopment. For example, one who believes that, once differentiation is 

 initiated, regression is impossible is likely to ignore appearance, structure, 

 and function of certain cells and to regard them as undifferentiated be- 

 cause they differentiate into new organs in reconstitution. The so-called 

 ''formative cells" in certain organisms are examples. Others, who believe 

 that dedifferentiation is possible, are likely to regard such cells as more 

 or less differentiated and as undergoing dedifferentiation in the activa- 

 tion and new development of reconstitution. 



It is an interesting question whether differentiation is entirely cytoplas- 

 mic or may involve the nucleus. Certainly, the nucleus takes on many 

 different forms and appearances in different cells. It becomes polymor- 

 phic, sometimes highly branched, in other cells much condensed, and its 

 staining properties may differ greatly in the same cell at different times. 

 The nucleus of the ovarian oocyte, for example, usually differs greatly 

 in appearance and staining from that of maturation and later stages. 

 Nuclei of most spermatozoa differ greatly from other nuclei of the same 

 species. Certain blastomeres of Ascaris undergo diminution of chromatin 

 in early cleavage stages; and following this change, number of chromo- 

 somes is greater, size less, and appearance very different. Moreover, local- 

 ization of diminution in certain cells in relation to centrifuging and dis- 

 permy indicates that regional cytoplasmic differences determine which 

 nuclei undergo this change and which do not (Boveri, 1910&; Hogue, 1910). 

 Whether these and many other nuclear changes appearing in the course 

 of development of gametes and other cells are to be regarded as differenti- 

 ations is at present largely a matter of opinion. A cytoplasmic environ- 

 ment seems to be necessary for long-continued nuclear life, and the 

 nucleus is certainly not wholly insensitive to change in this environment ; 

 that nuclear differentiation may be induced by the cytoplasm in some 

 cells seems probable, and perhaps nuclear self -differentiation is possible. 



The question of the basis of differentiation has interested biologists 

 since the study of development began. The Roux-Weismann theory of 

 qualitative nuclear division as the basis of differentiation is now discarded. 

 In the ordinary mitosis each daughter cell is supposed to possess the 

 same genie constitution as its parent. As Morgan once put it (1919, 

 p. 241), "each cell inherits the whole germ plasm." But cells and cell 

 groups become increasingly different from each other in the course of de- 

 velopment. How is this possible? 



Undoubtedly, there is interaction between nucleus and cytoplasm, and 

 supposedly different genes become activated or in some way come to 



