CHAPTER IV 



CHROMOSOMES AND GENES 



""^^ T OT only do the chromosomes pass through a 

 ^L series of manoeuvres that go far toward supply- 

 1 ^ ing a mechanism for the theory of heredity, but 

 from other sources evidence has accumulated supporting 

 the view that the chromosomes are the bearers of the 

 hereditary elements or genes, and this evidence has 

 steadily grown stronger each year. The evidence comes 

 from several sources. The earliest indication came from 

 the discoveiy that the male transmits equally with the 

 female. In animals, the male contributes, as a rule, only 

 the head of the spermatozoon, which contains almost ex- 

 clusively the nucleus composed of the condensed chromo- 

 somes. Although the egg contributes all the visible proto- 

 plasm of the future embryo, it has no preponderating 

 influence on development, except so far as the beginning 

 stages of development are determined by the egg proto- 

 plasm that has been under the influence of the maternal 

 chromosomes. Despite this initial influence, which can be 

 entirely ascribed to the previous influence of its own 

 chromosomes, the later stages of development and the 

 adult show no preponderance of maternal influence. 



This evidence from the mutual influence of the two 

 parents is not, however, in itself convincing, for, dealing 

 with elements that are ultramicroscopical, it might be 

 claimed that the sperm contributes something more than 

 its chromosomes to the future embryo. In fact, in recent 

 years it has been shown that visible protoplasm elements, 

 the centrosomes, may possibly be brought into the egg by 



