140 DIVISION OF LABOR 



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

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

 uew 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 as the result of the 

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

 An animal embryo 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 ciHa 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 fertilization similar in 

 plants and in animals? 



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



Problem JIG: To study 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 



