476 PATTERNS AND PROBLEMS OF DEVELOPMENT 



First, there is the interesting question: What is the nature of an embry- 

 onic axis without regional character? Second, the foreign inductor does 

 not always evoke an embryonic axis; it may induce merely a mass of tis- 

 sue or an unordered complex; and when an axis does appear, it usually 

 coincides with the host axis. Another interesting question is: How can a 

 piece of some tissue — for example, liver — induce an embryonic axis? 

 Third, according to these authors, an axis without individuation may 

 range from a neural tube with notochord and somites in proper position, 

 but one end not distinguishable from the other, to histologically differen- 

 tiated neural, chordal, and mesodermal cells without definite order. But 

 development of neural tube with somites and notochord in normal rela- 

 tions represents a very definite axiate pattern. Even if the two ends are 

 not distinguishable as different regions, it probably did not arise all at 

 once; and if it did not, a pattern with regional character of some sort must 

 have been present. The concept of an embryonic axis without regional 

 character seems somewhat metaphysical. Does not an axiate pattern, par- 

 tial or complete, constitute individuation? 



The concept of an individuation field originating from the inductor in 

 normal development (Waddington and Schmidt, 1933) also presents difii- 

 culties. The inductor does not originate embryonic pattern; it is a part 

 of that pattern. In normal development it does not determine the polar 

 axis of the embryo but merely plays a part in determining certain develop- 

 mental events in relation to that axis. Does not the pattern of the whole 

 amphibian egg constitute the individuation field, within which orderly 

 changes precede and bring about invagination and induction? 



CHEMICAL ASPECTS OF THE INDUCTOR PROBLEM 



That induction may be due to a chemical substance was suggested by 

 Spemann and Mangold (1924). The later discoveries that dead tissues and 

 tissue extracts, when associated with a more or less solid carrier, can in- 

 duce, and that boiling, treatment with alcohol, ethyl ether, acetone, ace- 

 tone followed by several days in water, or glacial acetic acid, does not de- 

 stroy inducing power and in some cases increases it, seemed to indicate 

 that any inducing substance present in the material is not soluble with 

 these treatments, though it must presumably reach the overlying ecto- 

 derm in some way. Further experiment showed, however, at least for 

 acetone, that both the extracted residue and the acetone extract could 

 induce.^'' 



^■'Spemann, Fischer, und Wehmcier, 1933; Fischer und Wehmeier, 1933(7, b; Wehmeier, 

 1934; Holtfreter, 1934a, b. 



