ANOTHER LINE OF EVOLUTION 59 



parent colony, each becoming an independent Gonium. 

 Sexual reproduction also occurs; some of the adult cells 

 break away from the colony, come to rest, then divide 

 into eight gametes, each having two flagella. The 

 gametes are of equal size, so that they give no indication 

 of sex- differentiation. They conjugate in pairs, form 

 zygotes, which in due season give rise to new colonies. 



Gonium is practically a colony of cells of the Chla- 

 mydomonas type; it represents an attempt to produce a 

 multicellular plant by the grouping of hitherto inde- 

 pendent cells. In Pandorina we have a plant founded 

 on similar lines, but showing an advance on Gonium. 

 Pandorina, also a very small plant, is found in ponds; 

 it is a colony of sixteen wedge-shaped cells forming a 

 sphere. Each cell has chlorophyll, a red pigment spot, 

 and a pair of flagella, and the colony is invested in a 

 gelatinous membrane. The plant swims with a rolling 

 motion. Daughter-colonies are produced asexually as 

 in Gonium. Colonies of gametes are also formed, usually 

 eight-celled; the gametes acquire flagella, escape from 

 the membrane in which they are originally invested, 

 swim about, and ultimately conjugate. But it happens 

 that a number of parent colonies of various sizes produce 

 gametes simultaneously. The resulting gametes are of 

 various sizes, and it is said that the largest gametes 

 never fuse in conjugation; they become almost passive, 

 and gametes of small size actively seek them out and 

 pair with them. But, with the omission of these large, 

 almost passive gametes, others of equal or unequal size 

 conjugate freely. Thus, we have in Pandorina a most 

 interesting illustration of the development of sex- differ- 

 entiation, for within the life-cycle of a single species 



