456 THE MAINTENANCE OF SPECIES 



of future advance is concerned with the direct analysis of the germ- 

 plasm itself, that is, of the basic chemical materials (genes) out of 

 which somatoplasms are derived. This has been made all the more 

 possible within the last half century by the increased efficiency of 

 greatly improved microscopes and microtomes, and through the 

 development of staining technique by means of aniline dyes which 

 render visible and differentiated microscopic details of structure that 

 were formerly unseen. 



Chromosomes 



Every germ cell, as well as each of the somatic cells that are the 

 building stones of the body, contains a nucleus, within which, at 

 certain times in the life cycle of the cell, chromosomes may be seen. 

 These structures stain more deeply with certain dyes than do other 

 parts of the cell, thus becoming visible under the microscope. 



It is doubtful that Mendel ever saw chromosomes, for it was not 

 until the late seventies, after his scientific career was practically over, 

 that the invention and development of aniline dyes made possible 

 their discovery. Each pair of chromosomes has a characteristically 

 different shape and size, whereby it is usually possible to distinguish 

 them from every other pair. Chromosomes, moreover, retain their 

 specific identity, in spite of the fact that they may change their form 

 temporarily, or for a time disappear entirely from view. When 

 germ cells undergo maturation to form their gametes as a preliminary 

 to fertilization, the total number of chromosomes in each cell is 

 reduced to one half. An entire pair is never normally eliminated, 

 although this sometimes occurs under abnormal circumstances 

 {non-disjunction). The result is that ordinarily there is left behind 

 one complete outfit of all the chromosomes characteristic of the 

 species, with their determinative genes, both in the mature egg and 

 the mature sperm. As pointed out previously, fertilization restores 

 pairs of chromosomes and then ever afterwards, by means of the 

 meticulous machinery of mitosis, these pairs are handed on to all 

 subsequent cells of the body that arise from the fertilized egg. 



One of the evidences that chromosomes play an important part in 

 heredity lies in the fact that they are the only parts of the germ cells 

 in which the two sexes contribute equally to the formation of the 

 fertilized egg in animals, or ovule in plants, that initiates a new 

 individual. It is common observation that each parent in the long 

 run is equally responsible for hereditary traits in the offspring, and 



