234 Mechanism of Mendelian Heredity 



species is characterized by a definite number of 

 chromosomes, e. g.^ 



man (probably) 24 corn 20 



mouse 20 evening primrose 7 



snail {Helix hortensis) ... 22 nightshade 36 



potato beetle 18 tobacco 24 



cotton 28 tomato 12 



four o'clock 16 wheat 8 



garden pea 7 



In the fertilization of the egg the number of chro- 

 mosomes is doubled (if we disregard for the moment 

 the complication caused by the X and Y chromosomes 

 which was considered in the previous chapter). It 

 was noticed by Montgomery that each chromosome 

 had a definite size and individuality, and he suggested 

 that homologous chromosomes existed in sperm and 

 egg and that in fertilization the homologous chromo- 

 somes of egg and sperm always joined and fused in the 

 special stage designated as synapsis, which will interest 

 us later. On the basis of this suggestion Sutton 

 developed the chromosome theory of the mechanism 

 of Mendelian heredity or segregation. 



According to this theory, all the cells of an individual 

 (inclusive of the egg cells and sperm cells) have two 

 sets of homologous chromosomes, one from the father, 

 the other from the mother. Before the egg and sperm 



^ Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. 

 B., Mechanism oj Mendelian Heredity. New York, 191 5, p. 26. 



