42 MENDELISM AND 



for all individuals of the same species, and that 

 before the gametes are ready for fertilisation two 

 cell- divisions take place, which result in the re- 

 duction of the number of chromosomes to half 

 the original number. When two gametes unite, 

 the specific number is restored. Since the male 

 gamete is very small and seems to contribute to the 

 zygote almost nothing except the chromosomes, 

 which carry with them all the characters of the 

 male parent, it seems a necessary conclusion that 

 the chromosomes alone determine the character 

 of the adult. There are, however, facts which 

 point to an opposite conclusion. 



Hegner,! for example, found that in the egg of the 

 beetle Leptinotarsa, which is an elongated oval in 

 shape, there is at the posterior end in the superficial 

 cytoplasm a disc-shaped mass of darkly staining 

 granules, while the fertilised nucleus is in the middle 

 of the egg. When the protoplasm containing these 

 granules was killed with a hot needle, development 

 in some cases took place and an embryo was formed, 

 but the embryo contained no germ cells. Here no 

 injury had been done to the zygote nucleus, but 

 these particular granules and the portion of pro- 

 toplasm containing them were necessary for the 

 formation of germ cells. In other experiments 

 a large amount of protoplasm at the posterior end 

 of the ovum was killed before the nucleus had begun 

 to segment, and the result was the development 

 of an embryo consisting of the head and part of the 

 thorax, while the rest was wanting. The nucleus 

 segmented and migrated into that part of the 



^ R. W. Hegner, 'Experiments with Chrysomelid Beetles,' m., 

 Biological Bulletin, vol. xx. 1910-11. 



