ONTOGENY 9 



gonidia, and there results a sharp distinction between repro- 

 ductive and vegetative cells (Fig. 9). Thus in Pleodorina, of 

 the thirty-two cells forming the colony, four anterior cells are 

 purely vegetative, the remaining twenty-eight are gonidial; 



FIG. 8. Reproduction in the colonial Flagellate, Pandorina morum. From 

 Hertwig, after Pringsheim. /. Free-swimming vegetative colony. II. Daugh- 

 ter colonies formed by the fissions of each individual in a colony similar to the 

 preceding. ///. Gamete formation in a colony formed as above. IV, V, VI. 

 Stages in the conjugation of gametes to form a zygote (VI). VII. Zygote after 

 growth to full size. VIII, IX. Production of swarm spores by the zygote. 

 X. Vegetative colony formed by fissions of the swarm spore. 



while in Volvox, where the fully developed colony may include 

 as many as twelve thousand or more individuals, only five to 

 fifty cells, scattered through the posterior half of the colony are 

 gonidial, the remainder being vegetative (Fig. 10). 



