SEXUALITY 667 



pseudoparadoxa. In all species the vegetative cells and gametes have a 

 haploid set of ten small dot-like chromosomes. Under appropriate con- 

 ditions, differing in different species and races, the vegetative cells pro- 

 duce or become gametes that copulate and form a diploid zygote cyst. 

 (In some species, vegetative cells function as gametes; in others, gametes 

 differ from vegetative cells.) Under certain conditions, maturation divi- 

 sions, restoring the haploid condition, take place in the cyst. The reduced 

 cells emerge from the cyst and each gives rise by vegetative multiplica- 

 tion to a clone. 



THE KINDS OF GAMETIC DIFFERENCES OBSERVED IN Chlamydomonas 



The basic problem of sexuality in unicellular organisms is whether 

 the copulating or conjugating cells regularly differ from each other. As 

 will appear at once, certain differences are found only in some species or 

 races, not in others; while other differences seem to be of general oc- 

 currence. 



Morphological differences between copulating cells or gametes. — In 

 C. coccifera (Moewus, 1937b), the copulating pairs invariably consist 

 of a large, nonflagellated gamete and a small, flagellated one. In C. 

 braunii, both copulants are flagellated, but one is always much smaller 

 than the other. In the remaining species, there is no regular morpho- 

 logical difference between copulating gametes. Nevertheless, in particu- 

 lar pairs of at least certain species (e.g., C. eugametos; Moewus, 1933), 

 one gamete may be as much as twice as large as the other, while in other 

 pairs of the same species no size difference appears. All possible kinds of 

 gamete combinations are found: large with large, large with small, and 

 small with small. Finally, in species like C. pseudoparadoxa (Hartmann, 

 1934), the gametes are regularly smaller than vegetative cells, though 

 the two gametes do not ordinarily differ from each other. 



Functional differences between gametes.- — In C. coccifera (Moewus, 

 1937b), the large gametes lack flagella, and the small ones retain them. 

 Consequently, the small gametes are more active and must move toward 

 the larger ones to accomplish copulation. Further evidence of the greater 

 activity of the smaller gamete appears during copulation, for its contents 

 regularly pass into the larger gamete. Less functional differentiation 

 appears in C. braunii; here both gametes are flagellated and active, but 

 during copulation the smaller gamete regularly empties into the larger 



