The Mastigophora 159 



Development of the Volvox colony (170, 234, 287) also involves the 

 formation of a spherical plakea with a phialopore and the inversion 

 ("extroversion") of the plakea through the phialopore to produce a 

 young colony (Fig. 4. 32, AG). This process of inversion in Volvox is of 

 some general interest in its similarity to a process which the "stomatoblas- 

 tida" undergoes in certain species of Grant ia and Sycon (73). In general, 

 the young colonies of Volvox escape separately after rupturing the sur- 

 rounding membranes, but those of V. aureus may emerge through a com- 

 mon pore in the wall of the colony. 



The details of sexual reproduction vary somewhat in different genera 

 and species. The gametes are similar in Gonium, but anisogamy is obvi- 

 ous in Eudorina, Payidorina, Platydorina, Pleodorina, and Volvox. Some 

 species of Eudorina and Volvox are heterothallic and others are homo- 

 thallic, althotigh the homothallic species of Volvox are protandrous. Some 

 of the heterothallic species of Volvox show sexual dimorphism involving 

 dwarf male colonies and large female colonies (259). Pleodorina is usually 

 heterothallic, with occasional homothallic variants. Such variation is 

 known also in the typically heterothallic Volvox aureus. Pandorina, 

 Platydorina, and at least some species of Gonium (255) are heterothallic. 



Sexual reproduction is preceded by differentiation of gametes. The de- 

 veloping macrogametes in Platydorina caudata (268) show no significant 

 increase in volume but they become denser in appearance and acquire a 

 yellowish tinge as they approach maturity. The flagella are retained and 

 the mature macrogamete emerges from the colony as an active flagellate 

 (Fig. 4. 32, L-N). The macrogametes escape from the female colony in 

 Pandorina also, whereas those of Eudorina, Pandorina, and Volvox 

 remain in place and are fertilized there. The development of micro- 

 gametes in Platydorina is similar to that of a daughter colony. Fission, at 

 the 32-cell stage, results in a curved plakea which soon undergoes inver- 

 sion and develops into a sphere. Flagella are developed and the spheroid 

 packets escape intact from the colonial matrix. Upon contact with macro- 

 gametes, the packet dissociates into its component gametes and fertiliza- 

 tion occurs (Fig. 4. 32, O, P). 



Microgametes of Volvox develop from enlarged cells resembling the 

 "gonidia." Development of microgametes is precocious in Volvox sperma- 

 tosphaera, V. weismannia, and several other species in that packets of 

 gametes reach maturity while young male colonies are still within the 

 parental colony. In other heterothallic species, mature packets develop 

 only after the male colonies emerge from the parent and grow to about 

 the size of female and asexual colonies. Volvox spermatosphaera differs 

 from other species in that every flagellate in the male colony may develop 

 into a packet of gametes. The mature packet is discoid in Volvox aureus, 

 V. spermatosphaera, and V. weismannia, while spheroid packets are de- 

 veloped in V. globator, V. perglobator, and several others (259), The 



