MULTICELLULAR ORGANIZATION: PHYLOGENETIC 



It is to be noted in this connection that here no cell has the power to 

 move independently of its fellows. 



An Example of Colonies of the Third Order; First Division: Volvox 

 globator. This creature is spherical, with all the cells confined to the 

 surface of the sphere so that the sphere has a cellular wall and a cavity 

 bearing no cells. A Volvox colony of ordinary size measures three hun- 

 dred microns in diameter. There is no definite number of cells in a 

 colony, so that the size varies greatly; a colony may measure over seven 

 hundred microns or even a thousand microns in diameter. Each cell has 

 all the parts described as belonging to a cell of Gonium pectorale. The 

 cells are united to each other by protoplasmic strands radiating from each 

 cell (Fig. 15). The colony revolves on a definite axis, and moves toward 

 one of its poles. The stigmata, which probably have to do with the recep- 

 tions of light impressions, are, therefore, always located in each cell to 

 face in the direction of travel as 

 much as their position in the cell- 

 colony will permit. It is of chief 

 importance here to note that 

 certain cells are modified to per- 

 form sexual functions. Any cell 

 of the mass may become a female 

 cell or give rise to a group of 

 male cells. The female cell is a 

 large spherical cell free from 

 flagella and provided with a 

 large nucleus and a highly 

 granular cytoplasm. This cell 

 is called an ovum (see Fig. 15). 

 Our figure shows the primitive 

 germ cells. By repeated divi- 

 sion, certain of these germ cells 

 become separate clusters of 

 small, colorless, spindle-shaped 

 cells with two flagella. These 

 are the male cells or sperm- 

 cells. It is to be seen here 

 that the ova are distributed 

 promiscuously throughout the 

 vegetative cells. There is ap- 

 parently a tendency to as- 

 semble cells differentiated in 

 a particular manner seen in the groups of sperm-cells; but this is only 

 due to their mode of origin. The clusters of sperms arising from 



end. 



FIG. 16. Part of the body of a sponge, Gran- 

 tia: mes., mesoderm; ect., ectoderm; end., 

 endoderm; spi., spicules. 



