DIVISION OF LABOR 



177 



partnership between algae and fungi), approximately 55,000 species 

 of fungi, and about 16,000 species of algae. 



Development of a Simple Animal. — Many-celled animals are 

 formed in much the same way as are many-celled seed plants. A 

 common bath sponge, an earthworm, a fish, or a dog, — each and 

 all of them begin life in the same 

 manner. In a many-celled animal the 

 life history begins with a single cell, 

 the fertilized egg. As in the flowering 

 plant, this cell has been formed by 

 the union of two other cells, a tiny 

 (usually motile) cell, the sperm, and a 

 large cell, the egg. After the egg is 

 fertilized by a sperm cell, it splits into 

 two, four, eight, and sixteen cells ; 

 as the number of cells increases, a 

 hollow ball of cells called the hlastula 

 is formed ; later this ball sinks in on 

 one side, and a double-walled cup of 

 cells, now called a gastrula, results. 

 Practically all animals pass through 

 the above stages in their development 

 from the egg, although these stages 

 are often not plain to see because of 

 the presence of food material (yolk) 

 in the egg. 



In animals the body consists of 

 three layers of cells : those of the 

 outside, developed from the outer 

 layer of the gastrula, are called ecto- 

 derm, which later gives rise to the skin, nervous system, etc. ; an 

 inner layer, developed from the inner layer of the gastrula, the 

 endoderm, which forms the lining of the digestive organs, etc. ; a 

 middle layer, called the mesoderm, lying between the ectoderm 

 and the endoderm, is also found. In higher animals this layer 

 gives rise to muscles, the skeleton, and parts of other internal 

 structures. 



HUNTER, CIV. BI. 12 



A moss plant. (J, the luoss body; 

 S, the spore-bearing stalk 

 (fruiting body). 



