194 ORIGIN OF SEX AND OF THE SOMA 



16 young ccenobia escaping from the nnother envelope 

 into the water and then growing to the full size. The 

 formation of gametes takes place by similar divisions, 

 but this proceeds to various extents, thus producing 

 gametes of different sizes. The individual gametes so 

 formed become loosened from the mother envelope, 

 all but the largest escaping singly and swimming about 

 individually in the water (Fig. 25, C). Gametes are pro- 

 duced at the same time from a number of ccenobia 

 lying close together, so that the thin mucilage becomes 

 full of swimming gametes of all sizes. These con- 

 jugate in pairs without reference to size (a, b, c), except 

 that the largest, which have remained where they 

 were formed, can only conjugate with the small active 

 gametes, which seek them out (Fig. 25, e). Spherical 

 zygotes are formed (d), which become covered with a 

 cell wall (/), from which the protoplasm eventually 

 escapes {g) as a flagellated cell, and this divides to 

 form a new coenobium. 



Here then we have an even earlier stage in the 

 evolution of sex than in Chlamydomonas monadina — 

 an intermediate condition between isogamy and hetero- 

 gamy — ^for there is no sharp division into two sizes, 

 the largesit gametes alone, which do not move from 

 their places, representing the female condition, the 

 smaller either acting as isogametes («) or as males (e). 



Eudorina and Pleodorina.' — These are forms with 

 spherical coenobiaj larger than Pdndorina, and with all 

 the cells (32 to 128) spherical and separate, forming 

 a single layer on the surface of the coenobium. Each 

 cell is of the Chlamydomonas type of structure. In 

 Eudorina all the cells of the coenobium are alike and 

 all take part in division to form new ccenobia, which 

 takes place just as in Pandorina. The gametes are. 



