590 GENETICS AND EVOLUTION 



tributions ot the male and female gametes are equivalent. Since the 

 nuclear components ot egg and sperm are equivalent, but the cytoplasmic 

 components are not (the sperm contributes essentially no cytoplasm to 

 the fertilized egg), this is a strong argument that the cytoplasm plays at 

 most only a minor role in the transmission of hereditary traits. 



The direction of coiling in the shell of the snail Limnaea peregra 

 is inherited, with right-handed coil dominant to left-handed coil. The 

 direction of coiling in an individual snail is governed, however, not by 

 its own genes, but by those of its mother. The results of reciprocal crosses 

 at first suggested that the direction of coiling was inherited by some 

 factor transmitted in the cytoplasm. The direction of coiling is deter- 

 mined by the orientation of the mitotic spindle apparatus in the first 

 two cleavage divisions. The orientation of the spindle is in turn deter- 

 mined by some action of the maternal genes on the unfertilized egg, 

 during its maturation within the ovary. This is clearly not an example 

 of cytoplasmic inheritance, but of normal genie inheritance. 



Some of the clearest evidence for cytoplasmic inheritance comes 

 from experiments in which an egg is fertilized, then deprived of the 

 female pronucleus before it fuses with the sperm. The embryo which 

 subsequently develops receives all of its nuclear material from the male 

 parent and all of its cytoplasmic material from the female parent. Such 

 embryos do not develop very far and usually cease development in the 

 late blastula stage. The German embryologist, Hadorn, fertilized an egg 

 from one species of salamander (Triton palmatus) with sperm from 

 another species (T. cristatus), removed the egg nucleus, grew the embryo 

 to the blastula stage, and then grafted some of the presumptive epidermis 

 of the blastula onto a normal larva of a third species, T. alpestris. The 

 transplanted epidermis was able to survive and developed into adult 

 skin characteristic of the species palmatus, which had contributed its 

 cytoplasm. It appears that the cytoplasm, not the nuclear genes, controls 

 the development of this trait in these animals. 



The investigations of T. M. Sonneborn of the inheritance of the 

 "killer" trait in paramecia (p. 162) provide another example of a char- 

 acter transmitted in the cytoplasm to succeeding generations. Only a very 

 few other traits are known in which there is fairly clear evidence that 

 cytoplasmic inheritance occurs: the susceptibility of fruit flies to carbon 

 dioxide poisoning, and the inheritance of certain respiratory enzymes 

 in yeast. 



291. Inheritance of Acquired Characters 



It was generally believed at one time that traits acquired by an 

 individual during his lifetime by some effect of the environment— by 

 use, training or accident— might be passed on to his offspring. It was an 

 important part of certain theories as to how evolution occurs (p. 698). 

 The development of the science of genetics has shown that this is theo- 

 retically improbable, if not impossible, and no experimental evidence to 

 support this concept has ever been found. Weismann, whose theory of the 



