VOLVOX 47 



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 disintegrate the zygotes fall to the bot- 

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

 the return of favorable conditions for development each zygote 

 loses its protective shell and undergoes cell division to form a 

 new colony. 



Examine Volvox colonies with a microscope. Use high 

 power and focus very carefully with the fine adjustment, at first 

 directing attention to the surface of the colonies and ignoring 

 the large, more deeply stained masses deeply imbedded within the 

 colony. The small darkly stained spots rather uniformly ar- 

 ranged in the wall of the sphere are the somatic cells. The less 

 deeply stained material between these cells in the inatri.r. Make 

 a drawing of a small portion of the colony wall showing the re- 

 lation of cells and matrix. 



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 adjoining cells. 

 Draw a small portion, greatly enlarged. 



In this species the relative numbers of somatic cells in the 

 wall of the colony is one of the easiest means of distinguishing 

 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. The darkly stained 

 masses on the interior of a male colony are clusters of micro- 

 gametes. By careful focusing the shape of the single microga- 

 metes within a cluster may be determined. Note that clusters 

 of microgametes near the equator of the sphere are shown in 

 side view. 



Make a drawing, 60. mm. in diameter, showing a male colony 

 in optical section. In such a drawing only a single ring of so- 



