72 ANIMAL LIFE AND SOCIAL GROWTH 



they persist as long as counts are taken. Clearly 



they belong to the more mature life of the cultures. 



Finally there are the Amoeba (Fig. 8) which do 



not develop in all cultures even if they are seeded 



FIG. 7. Vorticella. 



FIG. 8. Amoeba. 



into them at the beginning. Their appearance 

 is erratic; they may come in fairly early or not 

 until the fortieth day. They usually pass to a 

 maximum soon after they appear, and become 



