6o 



BIOLOGY AND HUMAN LIFE 



Fig. 31. Various kinds of animal cells 



/, flat epithelial cells, like those lining the cavity 

 of the abdomen in man and other animals; 2, co- 

 lumnar epithelial cells, like those lining the air 

 passages, with hairlike projections of protoplasm, 

 called cilia; 3, muscle cells, unstriped, like those 

 in the walls of the intestine and of blood vessels; 

 4, shapeless cells of naked protoplasm, like those 

 of Ameba or of white blood corpuscles; 5, cells 

 containing fat globules, like those in adipose tis- 

 sue; 6, bone cells surrounded by hard deposits of 

 limy material; 7, a nerve cell, or neuron (a, the 

 cell body with its branching outgrowths, or den- 

 drites; b, the longest outgrowth, the axon, ending 

 in c, the terminal branches). Each cell shows a 

 distinct nucleus 



48. Division of cells. 

 Every living thing has 

 a beginning as well as 

 an end. Among the one- 

 celled plants and ani- 

 mals a new individual 

 originates by the divi- 

 sion of the parent cell ; 

 or rather two individ- 

 uals originate in this 

 way (see Fig. 30). 

 Every larger plant or 

 animal, consisting of 

 very many or many 

 millions of cells, also 

 originates as a single cell 

 ( see section 35). The fer- 

 tilized egg cell divides 

 into two cells ; each of 

 these divides again ; and 

 so on (see Fig. 107). 

 As the cells remain 

 clinging to each other, 

 the whole mass in- 

 creases in size. 



The division of the 

 cell, whether it is a fer- 

 tilized egg cell, or a 

 one-celled organism, or 

 one of the many cells 

 making up a larger 

 plant or animal, takes 

 place in very much the 

 same way in all cases. 

 The nucleus divides 

 first, the little rods 



