THE GENERAL ASPECTS OF HEREDITY 13 



carried in the germ plasm of a strain for many genera- 

 tions, only to crop out again when a chance mating 

 of two heterozygous individuals takes place. 



It is not known how or when brachydactyly origi- 

 nated, but it probably occurred centuries ago, and pre- 

 sumably arose in the first instance as a mutation — i.e., 

 a sudden and probably spontaneous germinal change.* 



Fortunately, the segregation which takes place in 

 germ-cell formation can now be referred to definite 

 elements in the cells — namely, the chromosomes. 

 These are the elements of the nucleus whose con- 

 stancy in number and shape for each species of animal 

 and plant is one of the remarkable features of organic 

 structure. In the complicated process of mitotic 

 nuclear division, w^hich happens whenever cells 

 divide in the growth and development of the organism, 

 the essential fact is that they are split lengthwise, 

 so that each daughter cell contains in its nucleus the 

 longitudinal halves of every chromosome. Although 

 these bodies seem to merge in the resting nucleus 

 into a mass in which the outlines of the separate 

 chromosomes are lost, yet there are cases in which 

 the outlines can still be traced, each chromosome 

 forming a separate compartment or vesicle of the 

 nucleus. There is also evidence that the parts of 

 the various chromosomes maintain their special re- 

 lationships throughout the period between one 

 division and another even when visible boundaries 

 are lost, or, at any rate, that the}^ reassemble with 

 the same arrangement as they disappeared. There 

 is something, not at present understood, which main- 

 tains the unity of the chromosome as a persisting 

 structure, and determines the constancy of its relative 

 size and shape during mitosis in the various parts of 

 the organism. 



* For a discussion of the causes and nature of Mutation, see 

 Gates (1915, chap. ix.). 



