6 PLANT AND ANIMAL LIFE 



sion and is the final outcome of the growth and develop- 

 ment which have taken place in the cell. It marks 

 the passing of the adult, since its life is now turned over 

 to the young cells which represent it. 



Development from the germ cell. Nearly all ani- 

 mals, no matter how complicated their structure, be- 

 gin their life as a single cell, the germ cell. The germ 

 cell in the higher animals undergoes a series of rapid 

 divisions into smaller and smaller cells, until a mulberry- 



FIG. 4. Diagrams illustrating direct cell-division: a, cell-body; 6, nucleus; <?, 

 nucleolus. 



like mass is produced by this process of segmentation. 

 These cells all appear alike at first and are mixed to- 

 gether seemingly without order in a spherical cellular 

 mass. Later, the cells enlarge and are arranged in 

 three definite layers. 



The upper layer develops into the skin of the animal. 

 Through a curious infolding of the cells, it also forms 

 the nervous system. 



The middle and larger layer forms the greater part 

 of the body, namely, the muscles, bones, fibrous tissues, 

 heart, blood-vessels and urinary system. 



The lowest layer forms the lining of the gullet, stom- 

 ach, intestines and lungs. 



Development of organs from cells. As the few 

 primitive cells of the three layers change and grow into 

 the millions of special cells which make up the different or- 

 gans and tissues of the body, 1 and develop the power to do 



1 Differentiation. 



