458 Organisijis Are Products of Heredity mid Envhonvient unit ix 



Fig. 406 Mitosis in aii animal cell. Note two centrosonies near the micleiis in (/). 

 How many pairs of chroinosomes in {2)? What is happening to the chro77iosomes 

 in {3)? In (4)? How many chromosomes in each of the cells about to form in {<))? 



chromosomes have split lengthwise there 

 are twice as many chromosomes in the 

 cell as there had been originally. Next 

 the two halves of each chromosome 

 move apart until they are at opposite 

 ends of the cell. If you now counted the 

 chromosomes at each end of the cell, 

 you would find the same number at the 

 two ends; and if you had counted the 

 original number in the cell, you would 

 find as many chromosomes at each end 

 now as there were in the cell before it 

 began to divide. 



A delicate membrane then forms 

 around each group of chromosomes. In 

 the meantime, the chromosomes change 

 back into a chromatin network. Thus 

 two new nuclei are formed, each like the 

 nucleus from which it came. Each is 

 exactly like the nucleus from which it 

 came because it contains a string of genes 

 which came from the original genes. 

 When, shortly after this, the cell body 

 splits, cell division has been completed. 

 Two daughter cells have taken the place 

 of the fertilized tq^^ cell. And each has 

 the same kinds of chromosomes and 



genes as the fertilized t^^. The whole 

 process in some organisms takes no more 

 than half an hour. This complicated di- 

 vision of the nucleus, in which each 

 daughter cell receives one half of each 

 chromosome with a complete set of 

 genes, is known as mitosis (mit-toe'sis). 

 Now illustrate the mitotic changes in the 

 nucleus bv doing Exercise 2. 



You have just read about mitosis in 

 the division of the fertilized t^g cell. 

 When each of these daughter cells di- 

 vides later, and, in fact, in every cell di- 

 vision thereafter, the nucleus divides by 

 mitosis. There is only one normal excep- 

 tion to this; you will read about it soon. 

 Mitosis, or some modified form of it, 

 occurs in the cell division of the simplest 

 animals and plants as well as in the most 

 complex fomis. 



Studying mitosis in stained cells. A 

 stained slide can be made of some rapidly 

 orowiniT tissue of an animal or plant. If 

 this tissue is orrowing, some of its cells 

 are dividing. By huntino- over the slide 

 you can find several cells that have just 

 begun to divide, others that are near the 



