12 



first. As the sexual elements are equivalent to cells, they can 

 obviously only be produced in a multicellular body. 



As to the method by which the unicellular Protozoa 

 became multicellular Metazoa, it is probable that the passage 



Fig. 6. Magosphcera planula, Haeckel. Optical section of the colonial stage in the 



life-history. 



was eJBfected by some unknown colonial forms which may be 

 placed above Amceha, This is a more probable position for 

 the transition forms to occupy than the tops of any of the 

 Protozoa groups, such as Infusoria, or Foraminifera, or 

 Radiolaria would be, and the line stretching straight 

 upwards from near Amoeba may be supposed to pass through 

 the hypothetical compound Protozoa. In the development of 

 any one of the Metazoa, we see a unicellular organism 



Fig. 7. Blastula" stage in the development of a Metazoon. 



becoming multicellular (e.g., the blastula stage seen in 

 fig. 7), but the exact process difi'ers considerably in different 

 groups, and there is great difficulty in determining in which 



