THE HEREDITARY FACTORS AND DIFFERENTIATION 417 



These examples will serve to show the relations between the 

 sciences of genetics and of developmental physiology. Hitherto, 

 neo-Mendelism has been concerned mainly with the manoeuvres 

 of the hereditary units, and in large part with their manoeuvres 



Y///////////M \ 



y. 



50 



Y///////////////7\ 



M-H 



n 



I I I 



un 



Fig. 200 

 Shape-genes in gourds (see also fig. 199)- The abscissae give the form-indices 

 of the fruits, expressed as breadth/length ratios, running from very elongated 

 shape (small breadth/length ratio) on the left to very flattened (disc) shape on the 

 right. The ordinates represent frequencies. Top line, range of form-indices of 

 parent tvpes : 6, a long type (elongate) ; 50, a rounded type (sphere). The shape- 

 genes involved are A, B and /. A and B produce flattening, while / inhibits 

 their action. The constituent of line 6 is aaBBII, of line 50 AAhhii. The F^ is 

 intermediate and unimodal. The F.. is multi-modal: the extreme right-hand 

 group represents a new recombination comprising the ABU forms, resulting in 

 disc fruits. An F3 from one of these (bottom line) shows a sharp 3 : i segre-" 

 gation. The parent must have been AaBBii and the offspring 3 ABBii : i aaBBii. 

 (From E. W. Sinnott and D. Hammond, Amer. Nat. LXiv, 1930-) 



during the two cell-generations in which the reduction of chromo- 

 somes is brought about. It is now beginning to concern itself with 

 the mode of action of the hereditary units during the much larger 

 number of cell-generations involved in building up the adult 

 organism from the tgg: and this task it can only accomplish 

 satisfactorily in close contact with developmental physiology. 



27 



