200 PHYSIOLOGICAL GENETICS 



also as a determination stream. Other cases of the same type 

 will li.' mentioned in the oexl chapter (see page 229). 



In plants, we have already mentioned the determinative proc- 

 esses in Acctabularia (Haemmerling) dependent upon the flow 

 of a substance which may be called a determination stream. 

 Such a process in relation to definite genes would probably be 

 found if a genetic analysis of flower abnormalities were combined 

 with nnbryological experimentation. A not very clear hint that 

 such is the case may be derived from some remarks of Zimmer- 

 mann (1934) concerning hereditary traits in Anemone. Also, 

 mutations of the type of laciniate leaves might permit an analysis 

 in that direction. 



9. THE PROBLEM OF PATTERN 



In the preceding chapter, we were confronted with the problem 

 of pattern, the most important problem for both the geneticist and 

 the embryologist. Development is, of course, the orderly produc- 

 tion of pattern, and therefore, after all, genes must control 

 pattern. It is a formation of pattern when the axial organs in an 

 amphibian are determined; it is also formation of pattern when 

 the digits of a hand are laid down, as well as when two blastomeres 

 of different potency are separated. Most of our knowledge of 

 pattern formation comes from experimental embryology, which 

 is the science of analysis of pattern formation. We must now 

 try to find out whether genetics has furnished material that per- 

 mits an attack upon the problem of pattern in terms of gene 

 action. Before doing this, however, we ought to have definite 

 notions regarding this problem, as it emerges from the facts of 

 experimental embryology, according to our ways of interpreting 

 these facts. 



A. Pattern in Development 



Development, if considered apart from the component cells, 

 consists of a series of steps restricting the potencies of different 

 areas, which are thus consecutively subdivided into smaller and 

 smaller areas of more and more specialized determination. 

 For each part or organ or structure, then, sooner or later comes a 

 time whence its future is finally determined 1 or, in the language 



1 Harrison (1937) Wants the term final determination eliminated because 

 each determination is not final. But as it is final in normal development, 



