ZOOLOGY DIRECTIONS 



trate the wall of the female colony. When a microgamete unites 

 with a sexual macrogamete within a female colony a fertilized 

 egg called a zygote is formed. The zygotes thus formed do not 

 undergo development immediately but each becomes encased in 

 a firm, resistant membrane. When a female colony bearing these 

 protected zygotes disintegrates, the zygotes fall to the bottom 

 of the pond and remain there, inactive for some time. With the 

 return of conditions favorable for development each zygote loses 

 its protective shell and undergoes cell division to form a new 

 colony. 



7. Examine Volvox colonies with a microscope. Use low 

 power and focus very carefully. The large, deeply stained 

 masses on the interior of many of the colonies are the sex cells 

 or gametes. In most of the female colonies each mass is a 

 macrogamete. In the male colonies, each dark mass is a cluster 

 of microgametes. Note that the clusters of microgametes near 

 the equator of the colony are shown in side view. Look for 

 clusters showing end view. The small, darkly stained spots 

 rather uniformlj^ arranged in the wall of the sphere are the 

 somatic cells. The less deeply stained material between these 

 cells is the matrix. 



8. Make a drawing of a small portion of the colony wall 

 showing the relation of cells and matrix. 



9. At demonstration table examine specially prepared slide 

 of Volvox showing the fine protoplasmic threads which pass 

 through the matrix connecting each somatic cell with the adjoin- 

 ing cells. Draw a small portion, greatly enlarged, 



10. In this species the relative numbers of somatic cells in 

 the wall of the colony is one of the easiest means of distinguish- 

 ing the two sexes. Find a colony in which the distance between 

 two adjacent somatic cells is more than twice the diameter of 

 a single somatic cell. This is a male colony. Because of the 

 wide separation of the somatic cells such a colony appears much 

 lighter in color than the other colonies. 



