706 General and Applied Biology 



ever, such seeds are usually not sold because the yield is usually low and 

 grains are of variable size; (c) Two single-cross hybrids, produced from 

 different homozygous strains, are now crossed, producing double-cross 

 hybrids, which produce higher yields of uniformly large seeds (Fig. 

 348). 



Original Corn Plant Original Corn Plant Original Corn Plant Original Corn Plant 



Self-pollinated Self-pollinated Self-pollinated , Self-pollinated 



(inbred) for seven (inbred) for seven (inbred) for seven (inbred) for seven 



generations generations generations generations 



Inbred Strain 

 A 



Inbred Strain 

 B 



Inbred Strain 

 C 



Inbred Strain 

 D 



(Furnishes Pollen) (Detasseled) 



\ / 



Single-Cross Hybrid 



(AXB) 



(Detasseled) (Furnishes pollen) 



Single-Cross Hybrid 



(CXD) 



(Detasseled) 



Double-Cross Hybrid 



(AXB) (CXD) 



(Furnishes pollen) 



Seed for Commercial Planting (Fig. 348) 



Other genetic improvements in plants include fiber length in cotton, 

 sugar content of melons, yellow color of peaches, resistance to diseases 

 in plants (wheat rust, corn blight, oats smut, tomato wilt, etc.), resist- 

 ance of plants to pests (melon aphids, wheat Hessian fly, grape phyl- 

 loxera, etc.), seedless grapes, improved tobacco plants, etc. 



The genetic improvements in animals are extensive as shown by the 

 dev^elopment of poultry resistant to white diarrhea {Salmonella pul- 

 lorum) , resistance to abortion in rabbits, increased egg production by 

 developing earlier maturity in fowls, the production of platinum (silver- 

 blu) minks, hornless (polled) cattle, increased butterfat in milk, im- 

 proved qualities in race horses, better meat qualities in turkeys, improve- 

 ments in various breeds of dogs and cats, etc. (Fig. 349). 



17. Production and Maturation of Germ Cells. — Since a great amount 

 of the process of germ cell production, maturation, and fertilization deals 

 with the various phases of inheritance, it is discussed in this chapter. 



Weismanns theory of the continuity of germ plasm, states that the germ 

 plasm is transmitted from one generation to the next, or even many fu- 

 ture generations, in a continuous and uninterrupted manner. The body 

 cells (somatoplasm) arise from the germ plasm at the proper time and 

 become specialized for their various bodily functions. Body cells thus 

 can arise from germ plasm, but germ cells or germ plasm cannot arise 



