6 Genetics, Cells, and Chromosotnes 



treated in this manner is readily stained by certain dyes, some 

 of which stain one part of the cell and not others. The parts 

 so stained stand out in marked contrast to the rest of the cell, 

 and their structure is much more easily observed than it is in 

 the living condition. 



In the resting nucleus is always found the karyolymph or 

 nuclear sap, a clear fluid consisting mainly of proteins. In fixed 

 and stained nuclei, the nuclear membrane is generally stained 

 but the nuclear sap appears as an unstained or very lightly 

 stained background inside the membrane. Superimposed on 

 this background are the chromatin reticulum and one or more 

 nucleoli, both generally stained very deeply. 



In fixed and stained slides, the chromatin reticulum has the 

 appearance of a network and is composed of numerous very long 

 and extremely thin threads, in loose coils. These threads are the 

 chromonemata. When the cell divides another substance, the 

 matrix, condenses on these threads and the chromonemata and 

 matrix together form the chromosotnes, the most important 

 nuclear structures for the geneticist as they contain the genes. 



In the resting nucleus, the chromosomes are individually not 

 distinguishable, but they become identifiable as the cell divides. 

 During cell division it is clear that they exist in definite num- 

 bers which are the same not only for all the cells of a given 

 plant or animal, excluding certain reproductive cells, but also, 

 with certain exceptions, for all the individuals of the same species. 

 For example, all the somatic (that is, body or nonreproductive) 

 cells of the fruit fly, Drosophila melanogaster, normally have 8 

 chromosomes, whereas those of man have 48. Similarly, cells 

 of all normal maize plants have 20 somatic chromosomes, cells 

 of the garden pea have 14, and those of the onion have 16. 



Division of Plant Cells 



All cells come from preexisting cells by division. The term 

 cell division includes the division of both the nucleus and the 

 cytoplasm, either of which may divide even if the division of 

 the other does not occur. The division of the nucleus is called 

 mitosis or karyokinesis and the division of the cytoplasm cyto- 

 kinesis, but the use of mitosis for the entire process is not un- 

 known. It is customary to divide mitosis into four or five steps 

 which mark definite turning points in the process. Accordingly, 



