THE MUTATED GENE 221 



(Twitty and Elliott, 1934). Twitty explains this regulation 

 by the assumption that the assimilative capacity of younger, 

 faster growing cells is larger than that of older ones, which would 

 mean that they will take out of the available nutritive material 

 a larger share, and vice versa. If this is true, it would follow that 

 the constant k of the heterogonic growth formula controls the 

 avidity of a group of cells to apportion and assimilate a share 

 of the total available food. Regulation then would mean 

 formation of a certain equilibrium in the apportioning capacities 

 of these cells. It is clear that such results do not change the 

 picture derived from the work in physiological genetics but that 

 they add to it a new intermediate step between gene and char- 

 acter: the value of k, controlled by gene action, becomes the 

 ability of cells to secure a definite share of available food. 



Obviously, this is only one of the possibilities. It is generally 

 known that hormones play a decisive role in growth, e.g., the 

 molting hormones of insects (Buddenbrock, Wigglesworth), the 

 thyroid and thymus in vertebrates. No doubt, heterogonic 

 growth may also be controlled by hormones, as all the facts 

 concerning thyroid and metamorphosis, thymus and precocious 

 differentiation, sex hormones and growth of secondary sex 

 characters prove. There can be no doubt that among the prod- 

 ucts of genie action that control the growth pattern, hormones 

 of all the types discussed on page 181 play an important role, 

 including also all types of evocator substances. 



2. Symmetry. — Wherever pattern is involved, growth pattern 

 or otherwise, the problem of symmetry will come in sooner or 

 later. Usually, the patterns on both sides of the body are alike, 

 but sometimes they are not. Such an asymmetry may be due 

 to simple fluctuations of the same process occurring on both 

 sides in development. It may also be of a typical hereditary 

 type, which may range from an inclination to asymmetrical 

 distribution to such extreme asymmetries as in the claws of 

 fiddler crabs or other Crustacea. It is known that the problem 

 of symmetry looms large in experimental zoology. There is the 

 problem of the primary symmetry in eggs, the formation of the 

 axes of the embryo which is studied wherever experimentation is 

 made on the early stages of development. There is the problem 

 of handedness as evidenced in innumerable aspects of form and 

 function (see Ludwig's monograph, 1932) . There is the statistical 



