76 THE ELEMENTS OF VEGETABLE HISTOLOGY 



is accomplished. The details of mitosis or indirect 

 nuclear division are but briefly described in the follow- 

 ing statements. The network of linin threads is 

 unraveled into a definite number of semicircular 

 filaments each of which is covered with chromatin 

 bodies. These filaments are now termed chromo- 

 somes and are so arranged that the curved portion of 

 each is toward the center of the nucleus. Centrosomes 

 or attraction bodies are established at opposite sides 

 of the nucleus, forming slight thickenings termed 

 polar caps. A series of delicate fibers extends between 

 the polar caps. A second series of fibers are attached 

 to opposite sides of the chromosomes, each of which 

 has previously begun to separate longitudinally. 

 Contraction of this second set of fibers completely 

 separates each chromosome into two segments. The 

 individual segments representing a single chromo- 

 some are drawn toward opposite polar caps. The 

 group of chromosomes at each polar cap becomes 

 invested with a membrane and each group forms the 

 nucleus of a cell. Formation of walls to surround 

 the cells, of which each of these nuclei form the center, 

 begins when the chromosomes are in the center of the 

 old nucleus. The forerunner of the new cell wall 

 is formed at certain points by a thickening of the 

 fibrils extending between the polar caps. These 

 thickened portions form a structure which extends 

 across the cell and which is ultimately covered by 

 layers of cellulose produced by the cells which have 

 just undergone division. The new cells are furnished 

 with chromoplastids by direct division of these pres- 

 ent in the parent cell. 



