200 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



outnumber the germ cells. For example, V. perglobator, a species 

 common in the United States and very similar to the European 

 form V. globator (Fig. 106), is a colony of several thousand cells 

 arranged at the surface of a sphere the interior of which is occupied 

 by a watery fluid. Each somatic cell consists of a central mass 



Fig. 106. — Volvox globator. 



A, one-half of the spherical colony. B, fertilized macrogamete or ovum, v.ith male and 

 female nuclei uniting, and protective shell secreted after entrance of the sperm nucleus into 

 the cytoplasm of the ovum. C, microgamete or spermatozoon. Ov, ovum; S.C., somatic 

 cells; Sp. 1-2-3, developing spermspheres. 



of protoplasm containing a nucleus and connected with the proto- 

 plasm of neighboring cells by radiating strands that perforate the 

 cell walls. Each somatic cell further possesses two flagella, by 

 means of which the colony is propelled, a red pigment spot, and 

 a contractile vacuole. The germ cells of the volvox, which are 

 much less numerous than the body cells, become differentiated 

 from the general mass of somatic cells as the colony develops. In 

 their earlier stages they appear as cells of larger size, projecting 

 from the inner face of the somatic layer but retaining their connec- 

 tions with the outer surface (Fig. 106 A). The macrogametes, or 

 ova, as they are sometimes called, are formed by an increase in size 

 which is principally due to the formation of nutrient material in 

 the cytoplasm; the rnicrogametes, or spermatozoa, arise by the 



