beautifully illustrated by removing small 

 portions of the cytoplasm of an egg and 

 following the nucleated part in subsequent 

 development. Such an egg will produce a 

 normal embryo. However, if a chromosome 

 is removed from the nucleus of an egg cell, 



THE CONTINUITY OF CELLS 553 



will differentiate the chromosomes from 

 other regions of the cell. Excellent prepara- 



tions have been made of a large variety of 

 dividing cells. Among them the early em- 

 bryo of the whitefish affords some of the 

 best material (Fig. 22-1). Let us study this 



Fig. 22-2. Cell division can best be seen in embryonic tissues such as the early whitefish embryo (upper 

 left). This tissue is killed, stained, and placed in a paraffin block. Then very thin slices (6-8 microns) are 

 cut. These form a ribbon as shown here. When properly stained and mounted on a microscopic slide, 

 the activity going on within the individual cells can be studied. The five stages in mitosis are 

 schematically represented here. 



it probably will not develop at all. Even if 

 it should develop to some degree, the result- 

 ing embryo would be abnormal. Chromo- 

 somes are irreplaceable because the genes 

 of which they are composed are vital to the 

 life of the entire cell. 



The process of cell division can best be 

 seen in lar2:e cells that have been cut into 

 thin slices and stained with some dye that 



particular form as representative of animal 

 mitosis in general. 



Biologists have agreed, for convenience, 

 on five steps or phases which cells go 

 through during the duplication process 

 (Fig. 22-2). It must be remembered that 

 there is no hard and fast line of demarca- 

 tion between these phases because the proc- 

 ess is a continuous one. They merely denote 



