144 



UNIT AND DIVISION OF LABOR 



a. Development 



Observation. What is the beginning stage of existence of 

 a fern ? Moss ? Tulip ? Bird ? 



Conclusions. 1. Write a paragraph on the subject of Fer- 

 tilization. 



2. What must happen to the egg before it will develop ? 

 Note. You will recall that the sperm cells of the moss and 

 fern are motile, and that the egg is much larger and not 

 motile. Here is a fine example of a physiological (functional) 

 division of labor, which well meets the demand that one con- 

 jugating cell be very motile, and therefore comparatively small. 

 Thus the sperm cells can swim to the eggs in the water where 

 they are commonly set free. Another demand is that there be a 

 sufficient supply of food to enable the plant or animal embryo 

 to develop until such a stage of growth that it may care for itself. 

 So food is stored in the egg, thus making it comparatively 

 large and so hindering any great degree of independent move- 

 ment. Remember that sperm cells and eggs are single cells. 



Observations. 1. After fertilization the egg divides into 

 two parts, then these two divide, finally forming four cells, 

 then 8, 16, 32, etc., until it is much like a hollow ball. How 

 many layers of cells are there ? Are the cells now all of the 

 same size ? Note. The hollow ball stage is known as the 

 blastula stage. See Figure 42. 



2. Note that the hollow ball sinks in where the cells are 



FIG. 42. Stages in the segmentation of an egg, up to the formation 

 of the gastrula. 



