This would give four kinds of gametes, each getting one 

 or the other member of the two pairs of chromosomes, and 

 as each sex would have these four kinds of gametes the pos- 

 sible number of combinations in the production of new zy- 

 gotes would be 16. (Refer to page 116). 



(e)— Structure of Chromosomes 



Each chromosome is believed to be a linear aggregate 

 of chromatin elements, called chromomeres, and is reformed 

 at every cell division from the same chromomeres. It is 

 probable that the chromomeres in the pairs of parental 

 chromosomes meet in corresponding pairs and separate 

 when the chromosomes separate. Moreover, the chromo- 

 meres in a chromosome are different, each having a definite 

 place or locus. It is believed also that the chromomeres 

 represent Mendelian factors or genes. 



The recent work of Morgan and his associates with 

 Drosophila reveals quite definitely the structure of the chro- 

 mosome. The heritable factors, represented by loci, have 

 a definite linear arrangement in the chromosome, and when 

 a change occurs in some locus a corresponding change occurs 

 in the development of the individual. Moreover, the loci 

 act together a§ a system ,and a somatic change is the re- 

 sult of a change in the set of factors of the system. (See 

 page 123). 



The number of somatic unit characters is, therefore, a 

 multiple of the number of chromosomes, and may be group- 

 ed according to the number of the latter. 



Besides, the chromosomes have individuality, that is, 

 they vary in size, length, and shape. 



Much difference of opinion exists as to the material 

 substance of the chromomeres. Some physiologists hold 

 that they consist of enzymes, some hold that they are hor- 

 mones, and others that they are composed of a chemical 

 system of complex organic materials. But the opinions are 

 as yet mere guesses. 



(f) — Changes in Chromosomes 



As has been already noted, chromosomes may some- 

 t^imes change their constitution by 'crossing-over' of the 

 chromomeres. They may also be modified or injured 

 by subjecting them to certain chemicals, to high tempera- 

 ture, to electric currents, etc. 



Mutations may arise through "crossing-over of re- 

 cessive factors constantly present in the heterozygous 



139 



