KAKYOKINESIS 239 



your idea of the naiui-e of piotoplasin. What differences did you find 

 between the cells of yeast and those of green alga? In what ways do 

 they resemble each other? Tell the same of cells of protococeus and 

 of apple, or of other material studied. What is a vacuole? What 

 does it usually contain? Name two kinds of movements of protoplasm 

 within the cell -wall, and explain how each may be observed. Name 

 and describe two movements of naked protoplasm. Tell something 

 of the texture of cell-walls. What causes the markings found on cell- 

 walls ? Name five types of markings. Draw two figures to show 

 structure of bordered pits. Make a sketch of spiral, annular, and 

 scalariform markings. Name two methods of cell-multiplication. 

 Describe the process of cell-division. How does cell-budding differ 

 from cell-division? Name two methods of nuclear division. Which 

 is the more common method ? 



Note to PARAGRAPrf 393. — Karyokinesis (the indirect or mitotic 

 process of nuclear division) is an intricate subject. The details vary 

 in different plants, but the essential stages are as follows: 



Duringthe restingstage the nucleus is surrounded by a very deli- 

 cate but distinct membrane. Within this inclosure is an intricate net- 

 work of colorless {linin) threads bearing very numerous granules, which 

 in stained preparations are highly colored, and for this reason have 

 received the name chromatin. The network is surrounded by nuclear- 

 sap, and often incloses within its meshes a large body called the 

 nucleolus. As the time for division approaches the chromatin network 

 changes into a definite, much-coiled, deeply stained ribbon, in which 

 tlie granular structure is much less noticeable, and this in turn seg- 

 ments transversely into a number of parts called chromosomes. The 

 protoplasmic fibrils immediately surrounding the nucleus now grad- 

 ually converge towards two points lying on opposite sides of the 

 nucleus and at a slight distance from the membrane. This is accom- 

 plished in such a way that a spindle of nearly colorless threads is 

 produced, with the two previously mentioned points of convergence 

 acting as poles. Meanwhile both the nuclear membrane and the nu- 

 cleolus have disappeared, but whether these structures take part in 

 the formation of the spindle is yet an open question. Radiations 

 of pi-otoplasmic threads called asters sometimes occur around the 

 poles, and in a few lower plants, as well as in most animals, the pole 

 is occupied by a small spherical body termed a centrosphere. The 

 steps so far are known as the prophase stages. The chromosomes now 

 move to the equator of the spindle, where they an-ange themselves in 

 a definite manner, forming the so-called nuclear-plate {metaphase 

 stage). Each segment splits longitudinally, apparently on account of 



