io8 The Third and Fourth Generation 



cells which separate from the germinal material 

 differentiate into the body of the chicken, while 

 the undiEferentiated germ cells continue and 



Fig. 7. — At the left the old idea is illustrated that the chicken 

 gives rise to an egg from which comes another chicken that pro- 

 duces an egg, etc. At the right the modern conception is shown: 

 the egg gives rise to more egg-forming material and also to the 

 body of the chicken. The germ plasm is thus continuous. 



furnish the eggs out of which the next genera- 

 tion of chickens will develop. It is a mere 

 coincidence that the germ cells are contained 

 within the body of the parent. If this is true, 

 it is evidently difficult to conceive how changes 



