CHAPTER III 



GEEMINAL SELECTION 



To try to grasp the precise nature of the ordering of the deter- 

 minants in the germ-plasm is a hopeless task. But it is not to 

 be supposed that the determinants are located in the germ as 

 they are subsequently located in the fuUy formed organism. 

 Many groups of cells combine during the course of the embryonic 

 development, and co-ordinate their activities in the formation 

 of an organ, which originally possessed no co-ordination. The 

 ordering of the determinants in the germ-plasm need not corre- 

 spond to their subsequent ordering in the developed organism, 

 for the finished organism is not preformed in the germ, but 

 merely predestinated. 



If we observe the development of the limbs of those insects 

 which, in their larval phase, possess neither legs nor wings, we 

 find that during the larval period the Hmbs gradually develop 

 under the skin. In this case some definite little groups of cells 

 in the skin determine the formation of the limbs, and this group 

 of cells is the most important factor in the development of the 

 limbs. This cell group does not, however, determine the entire 

 leg of the insect, but only the hypodermis. The internal parts 

 of the leg, the nerves, tendons, and muscles, have their origin 

 in other groups of cells. It is the same with all organs of com- 

 plicated structure ; they are formed by the collaboration of the 

 forces of a number of difierent determinants, some of which define 

 one part, others another part, of the organ in question. 



This co-ordination of determining forces essentially involves 

 a combination of internal and external influences. Cells from 



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