Cell Division in Relation to Reproduction - 59 



Fig. 3-16. Photograph of conjugating Spirogyra. The 

 upper pair of filaments shows three pairs of cells at 

 the outset of conjugation; and the lower filaments 

 show a number of zygospores (dark oval bodies) 

 which result from conjugation. (Photograph retouched. 

 Copyright, General Biological Supply House, Inc.) 



times the same cells may act either as asexual 

 zoospores, or as gametes, as in the case of 

 Ulothrix, another filamentous green alga 

 (Fig. 3-18). In this plant the number of re- 

 productive cells produced by each cell of the 

 filament may vary from 4 to 64; the larger 

 the number, the smaller the size of the cells. 

 The larger spores soon settle down and de- 

 velop asexually into a new filament by re- 

 peated mitotic division (Fig. 3-18). The 

 smaller "spores" may also germinate in the 

 same way, but their growth is slower and the 

 filaments produced by them are less vigorous. 

 More frequently these smaller cells fuse in 

 pairs, thus acting as gametes and forming a 

 zygote (Fig. 3-18). After fertilization has oc- 

 curred, the zygote develops into a thick-walled 

 zygospore, as in Spirogyra. Likewise in 

 Ulothrix, the diploid zygote undergoes two 

 divisions, the meiotic divisions, just before 

 germination. At first the four resulting hap- 

 loid cells lie huddled together inside the old 

 cell wall (Fig. 3-18) but finally, after this en- 

 casement has disintegrated, each of the four 



haploid cells gives rise by mitosis to a sepa- 

 rate new filament. 



The evolutionary transition between isog- 

 amy, which is very common among simpler 

 organisms, and heterogamy, which reaches a 

 standardized condition in the eggs and sperm 

 of higher plants and animals, can be found 

 in a single group of free-swimming green 

 algal plants. This group, the Volvocaceae, 

 is made up of many species, but only three 

 species will be mentioned in the present con- 

 nection. 



All the Volvocaceae reproduce both sexu- 

 ally and asexually. The gametes formed by 

 Pandorina (Fig. 3-19) are of two sorts, one 

 slightly larger than the other. When fertiliza- 

 tion takes place, one of the smaller, or micro- 

 gametes, usually fuses with a larger, macro- 

 gamete. But sometimes two microgametes, or 

 two macrogametes, will come together. In 

 either case, a diploid zygote is formed, and 

 this gives rise to the new colony. Accordingly 

 it may be said that Pandorina shows the first 

 beginnings of a difference between the 

 gametes of the sexes. 



In Eudorina (Fig. 3-19) and Volvox (Fig. 

 3-19), the differentiation between the gam- 

 etes has developed much further. The macro- 

 gametes of Eudorina are several times larger 

 than the microgametes, although both gam- 

 etes are flagellated and motile. Moreover, 

 fertilization always involves a fusion of the 

 unequal gametes. But the climax of heterog- 

 amy is reached in Volvox (Fig. 3-19). The 

 macrogamete of Volvox, due to its relatively 

 large size and its inability to move, can prop- 

 erly be called an egg, and the very small 

 motile microgamete can truly be considered 

 a sperm. 



The differentiation between the sperm, 

 which is small and motile, and the egg, which 

 is large and nonmotile, represents an efficient 

 division of labor. The size and motility of 

 the sperm cell makes it an effective agent for 

 carrying the paternal chromosomes to the 

 egg. Usually the sperm are produced in very 

 large numbers and discharged near the eggs. 

 Consequently at least one sperm is almost 



