PROBLEM 4. The Effect of the Environment on the Organ'mn 



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A B C D 



Fig. 438 The cats shoivii are fe/nale cats. A sperm cell is shown about to fertilize 

 the egg (a^); other sperms fertilize the eggs of succeeding generations. What is the 

 shaded substance in each individual? Where is the somatoplasm? The egg (a^) and 

 the sperm cell can be spoken of as germ plasm. The resulting fertilized egg for?ns 

 more germ plasm like itself ajid it also for?fis so7fiatoplas7/7, the body of cat (B). 

 You can see that cat (B) did not produce its germ plasm. It only carries the germ 

 plasm which is a continuatioit of the germ plasm of cat (A) plus the sperm. 



Can the body affect the germ plasm? 



Remember that the germ plasm (pri- 

 mary sex cells) is held within the body or 

 soma; it is not produced by the soma. 

 Therefore Weismann argued that, even 

 though the soma may become changed, 

 even though it may acquire new charac- 

 teristics during the lifetime of the animal 

 or plant, these acquired characteristics 

 are not passed on to the germ plasm or 

 priman^ sex cells. This was a reasonable 

 conclusion for Weismann to arrive at. 

 And nowadays we still accept it as true. 

 In fact, since we have learned about mi- 

 tosis and genes we have all the more rea- 

 son for believing that acquired charac- 

 ters are not passed on to the new organ- 

 ism. Even in those lower animals and 

 plants which do not have distinct soma- 

 toplasm and germ plasm there is every 

 reason to believe that the genes are not 

 changed even though environmental 

 changes may occur in the cells. 



Weismann arrived at his conclusion as 

 the result of a famous experiment in 



which he cut off the tails of white mice 

 for almost twent)^ generations. As each 

 generation grew to maturity he measured 

 the tails of the mice before cutting them 

 off. He found that the tails neither dis- 

 appeared nor grew shorter. From this he 

 concluded that the environment changes 

 the body but that the germ cells are not 

 affected by changes in the body. In other 

 words, acquired characteristics (somatic 

 changes) are not inherited. 



There has been much criticism of 

 Weismann's experiment. It has been 

 pointed out that cutting tails produced 

 such a crude acquired character that no 

 change need have been expected. No one 

 believed that if a man lost his leg in an 

 accident his child born later would have 

 a shorter or a weaker leg. More evidence 

 on the question of inheritance of ac- 

 quired characters was needed. See Exer- 

 cise 8. 



More convincing evidence. For cen- 

 turies in many Chinese families the feet 

 of girl babies were tightly bound to keep 



