32 AN INTRODUCTION TO ZOOLOGY 



they are formed from the cytoplasm, and sometimes they arise 

 from the linin network in the nucleus. Linin does not, however, 

 differ greatly in chemical composition from the cytoplasmic 

 substance, and the spindle fibers are, therefore, always composed 

 of achromatic material. The nudeolus is apparently of no impor- 

 tance in mitosis; it degenerates during the early stages (Fig. 6, 

 G, H, n) and is reformed during the telophase (J). The two cells 

 that result from mitosis may be equal (J) or unequal in size. The 

 division of the cytoplasm appears to be of little importance, but 

 with rare exceptions there seems to be an equal division of 

 chromatin; this is apparently the essential process. 



Under ordinary conditions every animal develops from a single 

 cell, and, since the chromatin persists from one cell generation to 

 another, the chromosomes are considered by most zoologists to 

 be the bearers of hereditary qualities from parent to offspring. 

 Every species of animal has a definite number of chromosomes 

 that appear when the cells of its body undergo mitosis. Thus 

 sixteen are characteristic of the cells of oxen, guinea pigs, and man; 

 the grasshopper has twelve; the brine shrimp (Artemia), one 

 hundred and sixty eight; the round worm (Ascaris), four or two. 

 The last example illustrates one of the unusual cases in which two 

 individuals that are mutually fertile have a different number of 

 chromosomes. An even number of chromosomes is characteristic 

 of most animals, but recent researches have demonstrated that 

 some forms, particularly the males of insects, have an odd 

 number. 



(2) Amitosis. Although mitosis is considered the typical 

 method of cell division, the multiplication of nuclei by direct 

 division or amitosis is not uncommon. This process (Fig. 7) 

 does not have such a definite series of stages as mitosis, and is 

 much simpler. The nucleus elongates, and the nucleolus di- 

 vides, one half passing to either end of the nucleus. Sometimes 

 the nucleolus remains at one end, a new one being formed at the 

 opposite end. The nucleus divides by one of three methods: 

 (a) often the nucleus is pinched in two in the middle, or (b) a 



