306 THALLOPHYTES 
vitally related that the colony of Volvox may be regarded as a 
multicellular individual rather than a colony. 
Reproduction presents some interesting features. At first all 
cells of the colony are alike, but later considerable differentiation 
among cells occurs. Some cells of the colony enlarge and pass 
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Fic. 261.—Volvor. In the colony (Volvox aureus) the smaller cells bear- 
ing two cilia are the vegetative cells, the enlarged cells (a) contain sperms, 
and the enlarged cells (0), varying in size and stages of development, are 
eggs (x 300). Below and at the right is a sperm, and below and at the 
left is a odspore of Volvox globator. After West. 
into the hollow of the sphere where they form new colonies which 
escape and grow to adult size. Sexual reproduction in Volvox 
is heterogamous, for two distinct kinds of gametes are involved. 
Some of the cells enlarge, lose their cilia, and become filled with 
food. They are the female gametes or eggs. Other cells of the 
colony form numerous small motile gametes or sperms which seek 
the eggs and fuse with them. Fertilization, as this fusing is 
called since the gametes are differentiated into eggs and sperms, 
