INHERITANCE 193 



Structural Basis of Inheritance.— Just as it is possible to 

 identify in the reflex arc the structural basis of neuro-muscular 

 co-ordination in the higher animals, so it is possible for the 

 genetic physiologist to identify in the chromosomes the 

 structural basis of hereditary transmission in animals and 

 plants. It is probable that prevailing ignorance of the cellular 

 morphology of inheritance accounts in no small measure for 

 the neglect of Mendel's work by his contemporaries. By 

 the middle of the latter half of the nineteenth century the work 

 of Hertwig, Fleming, Strasburger, Boveri, Van Beneden, and 

 others had led to the recognition of the union of the nuclei 

 of the male and female gametes as the essential fact of sexual 

 reproduction ; of the constancy in number for every species 

 of the chromosomes or nuclear components in cell- division ; 

 and the maintenance of this constancy by the reduction of 

 the chromosomes to half the species-number in the production 

 of the gametes. By the beginning of the twentieth century 

 the studies of Strasburger and Sutton on the sizes and shapes 

 of chromosomes, the detailed study of the reduction division 

 and its antecedents by von Winiwarter, and the study of sexual 

 differences in the chromosome complex by McClung, whose 

 work was extended and elaborated by Stevens, Wilson, and 

 others, had accumulated sufficient evidence to locate in the 

 chromosomes the anatomical basis of MendeHan segregation, 

 and encourage the belief that the principles revealed by factorial 

 analysis were of widespread applicability. 



Let us now consider separately the conclusions derived 

 from experimental study in the light of microscopic knowledge 

 available to-day. 



I. Factorial analysis leads to the conception of material 

 units present in the fertilised egg in dupHcate, and segregating 

 before the formation of the gametes into maternal and paternal 

 components, one member of each pair and one only being 

 present in each gamete. As is well known, the chromosomes 

 in all animals and plants are present in the fertilised egg in 

 twice the number found to be present in the gametes . Further- 

 more, in many animals (and plants) from the most diverse 

 phyla, the chromosome complex of a species is characterised 



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