140 DIVISION OF LABOR 



NOTE. — In animals, as in plants, two cells, the sperm and the egg, unite to 

 form a fertilized egg. This cell will, under favorable conditions, develop into a 

 new animal. 



In animals, which is the larger, sperm or egg cell ? Which is the 

 movable cell? Suppose an animal, as a fish, laid its eggs in the 

 water, how might fertilization' take place ? 



NOTE. — The embryo of a plant {e.g., the bean seed) grows ae the result of the 

 division of the original fertilized egg into first two, then four, then eight, etc., cells. 

 An animal qmbryo develops in a similar manner. 



Arrange models in order to show development from a single cell 

 (the fertilized egg) to a hollow ball of cells, called the blastula 

 stage. (See figures above.) Note what happens next in develop- 

 ment. The cup-like structure is called a gastrula. How is the 

 gastrula stage formed? 



Note. — Most animals, including man, pass through the stages shown above. 



Suppose that all the cells had cilia in the blastula stage. How 

 would locomotion take place ? Suppose the hollow of the gastrula 

 is used as a food tube. Is there then any division of labor ? 



Conclusion. — 1. In what respects is fertihzation similar in 

 plants and in animals? 



2. What stages of development are alike in all animals? 



Problem 116: To stwdy the division of labor in tissues and 

 organs. 



Materials. — Charts and slides showing different kinds of 

 tissues, microscope. 



Method and Observations. — We have already found that cells 

 having the same structure and performing the same work form 

 tissues. Examples in our bodies are muscle tissue, nerve tissue, 

 connective tissue, etc. Does a blastula have more than one tissue? 

 A gastrula? Give reasons for your answers. 



Examine figure, page 179, Civic Biology, or slides showing different 



