CELL DIVISION 



61 



their individuality in and at the wall of this vesicle. In other organisms 



a film of protoplasm forms around each chromosome, 



and within the vacuole so created the chromatin spreads 



out in diffuse form (Fig. 39). In the example used in 



this figure these vacuoles do not fuse but remain 



separate. A third way of rendering the chromosomes 



indefinite in appearance, hinted at in the preceding 



account and earlier on page 26, is to have their chromatin 



spin out into fine threads, often branching, without the 



formation of vesicles in or around them. This method 



is combined with vesicle formation in the generalized 



illustration Fig. 35 (/, left). Occasionally, even when 



chromosome vesicles are formed, they are distinguishable 



as separate objects during the interphase (Fig. 40). 



Variations in Spindle and Cytosome. — A striking 

 variation in the spindle is the lack of any centrioles in 

 the cells of flowering plants (Fig. 41). In animal cells ^ ,^ ^ ^ 



. ^ \ o / Fig. 43. — Intra- 



they may be very minute but are usually present. The nuclear spindle in 

 rest of the spindle, that is, the fibers and radiating lines 1,^®, piotozoon 



•^ ' ' ° Euglypha. {From 



about the centrioles, may or may not be conspicuous. Wilson," The Cell," 

 In Fig. 42 the spindle fibers and the rays around the ^^^^^ Schevnakoff.) 

 centrioles are very conspicuous. But in the very fiat cells in the outer 



layer of the skin of salamanders there is 

 little or no sign of a spindle, even though 

 the chromosomes are sharply defined. 



The place where the spindle forms is 

 different in different organisms. In Fig. 

 35 it is shown forming outside, but near, 

 the nucleus. This is its usual origin. But 

 in certain protozoa and some multicellular 

 animals it forms wdthin the nucleus. In 

 ^f^^^^^^mlKi^M^^ such animals the spindle may be well 



developed and the chromosomes arranged 

 on it, or the chromosomes may even be 

 moving toward the ends of the spindle 

 (Fig. 43), before the nuclear membrane 

 disappears. 



With respect to the cytosome, the prin- 

 ,,,...,, .p cipal variation is the way in which the two 



the beginmng of the process, {rrom ^ -^ 



Dahigren and Kepner, "Principles cells produced by division are separated 

 of Animal Histologyn f^^^ ^^^ another in plants as compared 



with animals. Instead of dividing by means of a furrow around the 

 cell, plant cells form a group of nodules on the middle of the spindle 



Fig. 44. — Formation of the cell 

 plate in a dividing cell of the root 

 tip of the hyacinth. The thicken- 

 ings on the fibers of the spindle are 



