96 AN INTRODUCTION TO ZOOLOGY 



one to three swarm spores which escape from the cyst and swim 

 away (VIII-IX). Each swarm spore develops at once by binary 

 division a new colony of sixteen cells (X). 



The gametes of Pandorina are not equal in size as in Chlamy- 

 domonas, but appear to be of two kinds, some larger than others. 

 Where such a condition exists the smaller gametes are called 

 male cells or sperms, the larger, female cells or eggs. Sexual 

 reproduction consists in the union of a male cell with a female 

 cell, and certain investigators believe the larger gametes of 

 Pandorina tend to fuse with the smaller. If this is true, we have 

 in this organism the beginning of the evolution of sexual repro- 

 duction. 



In Eudorina elegans, another member of the family Volvo- 

 caceae, this distinction between male and female gametes is more 

 clearly seen. Eudorina is a spherical colony containing thirty- 

 two,- rarely sixteen cells which resemble the cells of Pandorina. 

 Reproduction by simple division takes place as in Pandorina. 

 This cannot go on indefinitely, for finally some colonies are 

 found whose cells have grown larger than usual. These are 

 female colonies, and the cells are macro gametes. Other colonies 

 produce microgametes; each of the thirty-two cells of these 

 male colonies divides, producing a flat plate of sixteen or thirty- 

 two spindle-shaped microgametes each with a pair of flagella 

 extending from its anterior end. When a plate of microgametes 

 encounters a colony of macrogametes it becomes attached to 

 it, the microgametes separate from the plate and make their 

 way through the gelatinous envelopes of the macrogametes, 

 thus fertilizing them and forming zygotes. 



It should be noted that the organization of Eudorina is more 

 complex than that of the other three species considered, and that 

 this advance in complexity is gradual. In Eudorina, also, the 

 small microgametes are perfectly distinct from the large macro- 

 gametes, and fertilization always consists in the union of a macro- 

 with a microgamete. This size difference is not so evident in 

 Pandorina. 



