THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 25 



to opposite sides of the nucleus, but remain connected by a spindle 

 of fine, colorless fibers, the achromatic spindle. This spindle, in 

 passing from one centrosome to the other penetrates the nucleus 

 and comes into close relationship with the chromatic network. 

 This latter structure, meanwhile, has arranged itself into a con- 

 tinuous filament which presently breaks into segments, called the 

 chromosomes (Fig. 7, 3). An interesting fact is that the number of 

 chromosomes into which the chromatin filament divides is the 

 same for all the cells of any given species. In the cells of the 

 guinea pig, for example, the number is sixteen. The number of 

 chromosomes, while characteristic, is probably not vitally signifi- 

 cant, since the cells of the onion have the same number as those 

 of the guinea pig. 



Each individual chromosome becomes attached to a fiber of the 

 spindle. Often the chromosomes take the form of a V, in which 

 case attachment is at the apex. The chromosome next splits 

 lengthwise, each granule dividing into equal halves (Fig. 7, 4). 

 By a shortening of the spindle fibers one subdivision of each 

 chromosome is drawn to one of the centrosomes and the other sub- 

 division to the other centrosome. The chromosomes then reunite 

 to form a continuous filament which, in turn, shapes itself into the 

 characteristic chromatin network of the resting cell nucleus. 

 The cell protoplasm divides, meanwhile, and the process is com- 

 plete. On the theory that the chromatin granules are the de- 

 terminers of the cell characteristics, this method of division in- 

 sures that the daughter cells shall resemble each other and the 

 mother cell very closely. As supporting this theory of the func- 

 tion of the chromatin is the interesting observation that during 

 the early stages of development, while tissue differentiation is in 

 progress, those cells that are destined to become the progenitors 

 of special tissues lose portions of their chromatin, by causing them 

 to dissolve in the cell protoplasm and disappear. The deduction 

 is that these specialized tissues are not going to need all the char- 

 acteristics of undifferentiated protoplasm and so disburden them- 

 selves as early as possible of those determiners for which they have 

 no further use. 



Significance of the Physiological Properties. Adaptation. If 

 we take the liberty of personifying Nature to the extent of ascrib- 

 ing purposes to her, we can say that the purpose of Nature with 



