22 PROTOPLASM 



daughter cells (Fig, 12g). This last statement has been tenta- 

 tively modified by recent work. The chromosomes are supposed 

 to contain centers of hereditary characters, the so-called genes, 

 arranged in an orderly and definite manner. If these genes are 

 to keep their respective places, the chromosomes must persist 

 as such in the resting nucleus, otherwise the reorganization of 

 the chromosomes at the next division would probably involve a 

 helter-skelter rearrangement of the genes. 



In plant cells, the spindle fibers thicken up at their mid-points 

 (Fig. 12/), forming the cell plate, which is the first indication of 

 the new cell wall (Fig. 12g). 



Fig. 13. — Fig. 14. — Spiral plant chromosomes. 



Chromosome (o from B. P. Kaufmann; b from K. 



"satellites." Sax.) 



The manner in which chromosomes get to their respective poles 

 is the subject of much discussion. It has been thought that they 

 are attracted magnetically, that they crawl or worm their way, 

 and that they are pulled by the spindle fibers. This last idea is 

 supported by evidence of a point of attachment of the fiber to 

 the chromosome. 



Chromosomes often show peculiarities in external form. 

 They may possess little satellites (Fig. 13). 



Recent work on chromosomes has shown them to be of spiral 

 structure. The chromosome was formerly thought to be a fine 

 emulsion of chromatin and other substances, or, what amounts 

 to the same thing, vacuolate in structure, until Kaufmann 

 discovered, or rediscovered, a spiral structure in them (Fig. 14). 

 The spiral structure of chromosomes is now generally accepted 

 as typical of plants, and possibly also of animals, although zoolo- 

 gists have given little attention to this feature of chromosomes. 



Studies on the structure of chromosomes by Painter and 

 Metz permit an interpretation of the inner arrangement of parts. 

 Painter has made a general map of the chromosomes of the 



