CELL DIVISION 79 



chromosomes retain their individuahty even during the period when 

 the chromatin is arranged in the form of a network, so that when the 

 chromosomes are reformed during the prophases each chromosome 

 contains the same chromomcres which it contained at the close of the 

 anaphase. According to this view certain chromosomes or groups of 

 chromosomes are responsible for certain hereditary characters and prob- 

 ably for certain physiological activities depending on the elements 

 contained in their chromomeres. If this assumption be correct, the 

 longitudinal splitting of the chromosomes with the resulting splitting 

 of the longitudinally arranged chromomeres, is the only method by 

 which the equal division of those substances which control cell activity 

 and heredity may be assured. It is worthy of note in this connection, 

 however, that while the exact halving of the chromosomes occurs in 

 the division of the somatic or body cells of animals, it does not occur 

 in the reduction mitoses of the germ cells, which are dealt with in de- 

 tail in Chapters X and XI. 



Excepting frequent differences between the sexes, the number of 

 chromosomes is constant for each species. Thus there are four chro- 

 mosomes in the nematode worm Ascaris megaloce'phala, eight in the 

 fruitfly, DrosopMla melanogastei^ 22 in one of the bugs, and according 

 to one investigator 47 in the male of the human species. ^^ 



Mechanism of Mitosis. — The form of the mitotic figure in the late 

 prophases, metaphase and the early anaphases, and particularly that 

 portion of it made of achromatic material has led to its comparison 

 with the lines of force revealed by the arrangement of iron filings in 

 the field of a horseshoe magnet. Such a comparison furnishes an excellent 

 illustration or model, but it offers little to the solution of the mechanism 

 of mitosis. Nevertheless, it is not impossible that mitosis is an electro- 

 magnetic phenomenon. 



One of the older views in regard to the forces operating in mitosis was 

 that the astral rays and spindle fibers of the achromatic figure were 

 protoplasmic threads which were capable of contraction. According to 

 this view certain fibers were attached to the chromosomes and the move- 

 ment of the chromosomes was due to pulling by these fibers. This con- 

 ception is not without support at the present time. Originally evidenced 

 only by observations on fixed and stained cells it has had within recent 

 years some support from the experimental side. A long series of ex- 

 periments in which dividing plant cells were subjected to solutions of 

 various poisons has shown that these achromatic structures act as if the 

 parts were made up of definite fibers which are thrown out of their normal 

 position by the action of the poison. Certain investigators claim to have 

 dissected out individual chromosomes from the dividing cells of salaman- 

 der testes and occasionally to have found a chromosome to which was 

 attached a minute protoplasmic thread which they considered to be a por- 



