FERTILIZATION 



585 



In all of these cases, presumably, maturation of nucleus occurs 

 to prepare it for union with its mate. In most cases maturation takes 

 place in the last two divisions of the nucleus, before formation of the 

 fusion nuclei or pronuclei. These two meiotic divisions are similar to 

 those characteristic of spermatogenesis and odgenesis in Metazoa (see 

 Sharp, 1934). The result of meiosis is the halving of the chromosome 

 number to the haploid condition, so that fusion of the two haploid 



Figure 140. Copromonas subtilis in hologamous copulation. A, vegetative form; B, 

 two individuals beginning to fuse anteriorly; C, cytoplasmic fusion well under way, 

 nuclei in heteropolar, second "reduction" division; D, zygote with synkaryon and single 

 flagellum. (After Dobell, 1908.) 



gametes will reestablish the diploid number which is characteristic of 

 the species. 



In the Mastigophora, syngamy has been described in very few forms 

 except among the Phytomastigina, in which it seems to be the general 

 rule. A typical case of hologamous copulation was described by Dobell 

 (1908) in the colorless phytomonad Copromonas subtilis (Fig. l40). 

 Two organisms which appear identical to each other and to ordinary 

 vegetative forms come together and partially fuse. This partial union 

 evidently acts as a stimulus to the nucleus of each gamont, for it pro- 

 ceeds to undergo two "maturation divisions" before fusing with its mate. 

 Of the two products of the first progamic division, one degenerates and 



