1915] 



MACDOUGAL MODIFICATION OF GERM-FLASM 257 



ontogeny, generally much later in the plant than in the animal, 

 and this is a matter which may be determined by the environic 

 agencies, the germ-plasm or meristem tract undergoes such 

 change of phase that instead of all of its separating elements 

 passing into somatic cells a few become reproductive masses 

 from which sexually specialized elements may be differenti- 

 ated, and in which the number of chromosomes, the metabolic 

 balance, degree of hydratation, auxetic energy and mechanism 

 of division suggest physico-chemical conditions widely dif- 

 ferent from those of somatic elements ; furthermore, the repro- 

 ductive elements are highly individualized. The meristem in 

 its myriad cells may at any moment present all of the phases 

 of growth and differentiation. The egg nucleus or the fer- 

 tilized egg, a single element of the plasma, may include the 

 fate of the individual and its unending line of progress, and it 

 may be affected in its entirety by agencies impinging upon 

 it. The reaction of such specialized cells to external agencies 

 would of course be different from those of the meristem tracts, 

 which are made up of plasmatic units of the most generalized 



form. 



The experiments of Tower with the Leptinotarsae, which 

 have been carried on under widely diverse conditions in 

 southern tropical Mexico, in the arid semi-tropical climate of 

 the Desert Laboratory, and under controlled conditions at the 

 University of Chicago, furnish a great series of cultures of 

 these beetles in which it is possible to demonstrate logically by 

 exclusion and analysis that certain climatic features, notably 

 moisture, may affect the germ-plasm, or the entire organism 

 when the germ-plasm is in a certain stage, in such manner as 

 to induce disturbances in hereditary lines. These experi- 

 ments show the vulnerability of the germ-plasm. 



That the germ-plasm is directly responsive to the action of 

 foreign substances which are introduced into the embryo-sac 

 was demonstrated when (early in 1905) I was so fortunate as 

 to hit upon an experimental method of treatment of the 

 ovaries of seed-plants which resulted in the formation of em- 

 bryos developing into individuals not entirely identical with 

 the parental types. The essential feature of the discovery 



