290 CELL-DIVISION 



formed in dough by the gas produced by yeast. The final 

 result is to reduce each chromosome to a ragged network. 

 These individual nets become closely associated in the 

 nucleus and presently it is no longer possible to tell where 

 one stops and another begins. In exceptional cases the 

 individual nets can be recognized at all times and so we 

 know that in these particular organisms each chromosome 

 maintains its own individuality at all times. Although 

 this cannot be directly observed in most plants and ani- 

 mals there is much indirect evidence to indicate that it is 

 true for them also. 



The Division of the Cell-Body does not occur until 

 after the nucleus has completed its division. About the 

 middle of the spindle, there is left a mass of fibers which 

 extend entirely across from pole to pole and are not 

 attached to the chromosomes. As they shorten the ends 

 are drawn in toward the middle and thickenings are 

 formed at the middle point. Eventually these touch and 

 spread out to form a sort of membranous plate across the 

 middle of the cell. A split appears in this plate and the 

 two halves separate slightly so that a cell-wall can be 

 laid down between and joined at its edges to the old wall. 

 When all this has been accomplished it remains only that 

 the two new cells should again grow to full size and either 

 become differentiated in their final form or else repeat the 

 division process. 



Summary. — The important points to bear in mind 

 are: 



1. That the nucleus contains a definite fixed number of 

 chromosomes for each and every kind of animal or plant. 



2. That each chromosome maintains its own identity 

 throughout all the changes of form. 



3. That each increases its substance through growth, 

 and divides lengthwise into two identical halves. 



4. That one half of each goes to each new nucleus. 



5. That the net result of the whole process is to guaran- 



