LECTURE XIX 

 THE GEKM-PLASM THEORY (continued) 



Co-operation of the determinants to form an organ : insect appendages— Venation 

 of the inseet-wing — Deformities in Man — Apex of the fly's leg — Proofs of the 

 existence of determinants — Claws and adhesive lobes — Difference between a theory 

 of development and a theory of heredity — Metamorphosis of the food-canal in insects — 

 Delage's theory — Keinke's theory of the organism-machine — Fechner's views — 

 Apparent contradiction by the facts of developmental mechanics — Formation of the 

 germ-cells— Displacement of the germinal areas in the hydro-medusoid polyps, a proof 

 of the existence of germ-tracks. 



It would be futile to attempt to guess at the arrangement of the 

 determinants in the germ-plasm, but so much at least we may say, 

 that the determinants do not lie beside each other in the same 

 disposition as their determinates exhibit in the fully-formed 

 organism. This may be inferred from the complex formative pro- 

 cesses of embryogenesis in which many groups of cells, which in their 

 origin were far apart, combine together to form an organ. Thus the 

 arrangement of the determinants in the germ-plasm does not corre- 

 spond to the subsequent arrangement of the whole animal, nor are 

 primary constituents of the complete organs contained within the 

 germ-plasm. The organ is undoubtedly predetermined in the germ- 

 plasm, but it is not preformed as such. 



Here, again, the history of development gives us a certain basis 

 of fact from which to work. Let us consider, for instance, the origin 

 of the appendages in those insects which in the larval state possess 

 neither legs nor wings, but exhibit a gradual emergence of these 

 structures from concealment underneath the integumentary skeleton. 

 In these cases, as I have already shown in regard to the wings, 

 the development of the limbs arises from definite groups of cells in 

 the skin. These must therefore be regarded as the formative, and 

 therefore as the most important and indispensable, parts of the 

 rudiments, and may be designated the imaginal disks, as I many years 

 ago proposed ^ (Fig. 89, ui and oi). 



But these disks of cells do not contain the whole leg, but only the 



' Die EnhoicMung der Dipteren, Leipzig, 1864. 



