76 MAN, — THE ANIMAL 



It is noted that the individual granules do not have 

 any specific relationships to one another except for 

 a brief period when the cell is dividing. (Com- 

 pare Figs. 20-22.) These chromatin granules, 

 in anticipation of the dividing of the cell, at cer- 

 tain intervals take a definite position and become 

 greatly altered in appearance. Fig. 20 shows 

 the granules arranged in bands. Each band is 

 double, due to the splitting of the chromatin gran- 

 ules. In Fig. 21, these same bands are shown. 

 Each one has become more compact and much 

 shorter. In one the double condition is still evi- 

 dent. At about this stage of growth, the nuclear 

 wall breaks down and the contents of the nucleus 

 and cytoplasm are more closely associated. Such 

 a condition is shown in Fig. 20. An important 

 fact to keep in mind at this point is that these 

 bodies retain their individuality after the nucleus 

 breaks down. The descriptive term now employed 

 for these bodies shown in Figs. 20-22, is the word 

 chromosome. 



It is known that there are a definite number of 



chromosomes for each species of animals; for 



example, man has 48, the frog 14, many snails 32, 



some worms 4, while one of the simplest animals 



(Paramecium) has more than 100. 



When the earliest students first began studying 

 the nucleus, they little dreamed of its complexity. 



