FERTILIZATION 589 



conclusive evidence for copulation, while Gross (1934) described copu- 

 lation of isogametes. It seems odd that the life history of this abundant 

 and spectacular species should still be a matter of such uncertainty. 



Sexual reproduction is widespread among the Phytomonadida. These 

 plant-like flagellates illustrate so nicely the gradations in sexual develop- 

 ment and differentiation that they have been favorite material for class- 

 room instruction. 



Chlamydomonas is a non-colonial genus among the species of which 

 both isogamy and heterogamy are found. In C. steinii, according to 

 Goroschankin ( 1891 ) , the flagellate divides into many isogametes within 

 a cyst. The gametes fuse, beginning at their flagellated ends, and zygotes 

 are formed which develop into resistant cysts. In C. hraunii, the same 

 author ( 1890) describes gametes of different sizes fusing in anisogamous 

 copulation. Besides being definitely though not pronouncedly smaller, 

 the microgamete is slenderer and more pyriform than the macrogamete. 

 In still another species the differentiation is still more striking. Some 

 individuals of C. coccijera (Goroschankin, 1905) are transformed di- 

 rectly into large nonmotile, egg-like macrogametes, while others divide 

 into relatively very small, flagellated, sperm-like microgametes. 



Chlamydomonas, therefore, illustrates possible stages in the evolution 

 of sex from isogamy, in which the gametes are smaller than vegetative 

 individuals but otherwise similar; through early differentiation of gam- 

 etes, wherein the gametes are only slightly though clearly differentiated 

 in size and therefore exhibit the very beginnings of anisogamy, to ex- 

 tremely well-differentiated heterogamy or oogamy, in which the micro- 

 gametes and macrogametes are almost typical sperms and eggs. Only one 

 more fundamental advance has been made in the evolution of sex in the 

 Metazoa, and that is the differentiation of the adult forms into male and 

 female individuals. Structural developments for the production and care 

 of offspring belong to a different category. 



It is unfortunate that so little is known of the maturation processes 

 in Chlamydomonas. It is not known where reduction occurs, much less 

 what the nature of the chromosomes and their behavior in reduction are. 

 Some investigators assume that because of the way in which the zygotes 

 of Chlamydomonas, Gonium, Pandorina, and so forth, behave at divi- 

 sion, reduction is zygotic. Dangeard (1898) found no nuclear reduction 

 taking place before fertilization in Chlamydomonas and suggested that 



