ONTOGENESIS 



149 



do not begin to develop until the microgametes have 

 matured and broken away from the colony, so that 

 little or no opportunity is afforded for zygosis to 



FIG. 59. Volvox globator, a large colonial flagellate: A, a sexually 

 ripe colony, showing microgametes (tf) and macrogametes (?), in vari- 

 ous stages of development; B, a portion of the edge of the colony highly 

 magnified, showing three flagellate cells united by protoplasmic threads, 

 and a single reproductive cell, rp ; st, stigma or " eye-spot " ; cv, con- 

 tractile vacuole. (From Bourne, after Kolliker.) 



occur between members of the same colony. The 

 fusion of the passive megagamete and the active 

 microgamete occurs within the colony of which 



