46 LABORATORY DIRECTIONS 



In Volvox is found a still higher type of colony life where 

 several hundred or even thousands of cells have become associ- 

 ated to form a single layer of cells over the surface of a sphere. 

 In such a colony some individual cells are commonly specialized 

 to carry on the reproductive function while all of the remaining 

 cells (somatic cells) are similar. These colonies live in ponds 

 and pools where they swim about freely by the action of the 

 flagella with which the somatic cells are provided, progressing 

 with a smooth rolling motion. Ordinarily in preserved material 

 used for study these flagella are not distinguishable. 



There are a number of different species of Volvox. These 

 differ among themselves in details of structure as well as in meth- 

 ods of reproduction. The material selected for this study belongs 

 to the species Volvox weismannia. In this species there are two 

 different methods of reproduction involving three different types 

 of individuals. The simplest and most characteristic method of 

 reproduction is found in the parthenogenetic female colonies. In 

 these, some of the cells become set apart as reproductive cells 

 and are called parthenogenetic macrogametes, arising in the col- 

 ony wall, they . increase in size until they are shoved into the 

 interior of the colony. Macrogametes of this sort are capable 

 of undergoing development directly without being fertilized. 

 Each gamete by repeated divisions forms a group of cells which 

 become arranged as a small sphere within the parent colony. Only 

 by rupture or disintegration of the wall of the parent colony 

 are these young colonies liberated. This method of partheno- 

 genetic reproduction continues as long as conditions are favorable. 



At certain times the young colonies instead of producing 

 parthenogenetic gametes develop into true sexual individuals. 

 Thus there are formed male colonies which develop only micro- 

 (jametes or male cells and sexual female colonies which produce 

 only sexual macrogametes. In these true sexual individuals 

 neither type of the reproductive cells is capable of undergoing 

 reproduction independently, for fertilization is necessary. When 

 fully formed the microgametes leave the male colony and pene- 

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



