158 SEX IN MICROORGANISA4S 



gametes proceeds and excystation takes place within 15 to 20 hours; 

 within 4 to 6 hours, after excystation, fertilization begins. Excysta- 

 tion, maturing of gametes, and fertilization may continue until 35 

 to 40 hours, but in most cases are completed in 24 to 30 hours, after 

 molting. Some of the cysts are egested just before or just after the 

 host molts. The same development takes place in cysts remaining in 

 the original host as in those egested and taken up by a newly hatched 

 nymphal host. 



After a haploid gametocyte encysts (Fig. I, 1), it produces two 

 unlike gametes. The first step is the production of two unlike daugh- 

 ter chromosomes by each chromosome in the nucleus. These daughter 

 chromosomes differ in staining capacity in Heidenhain's haematoxy- 

 lin, the male chromosomes staining somewhat more darkly than the 

 female (2). Separation begins during prophase of division, and a spe- 

 cial type of union insures that the male group of chromosomes sepa- 

 rates from the female group on the mitotic spindle (3). After cyto- 

 plasmic division, which produces two gametes (4), only slightly, if 

 at all, different in size, cytoplasmic differentiation of gametes takes 

 place. This may become initiated before excystation but usually be- 

 gins afterwards. 



At the posterior end of the female gamete, a ring of deeply 

 stainable granules is gradually formed. The clear area within this 

 ring may be everted as a cone, or retracted. It is apparently attractive 

 to the male gametes, in which dark granules are rather uniformly 

 distributed. A male gamete becomes attached to the fertilization cone 

 and follows this as it is withdrawn into the body of the female gamete 

 (5). Penetration by the male gamete is rapid, after which the fertili- 

 zation ring disappears and the cytoplasmic organelles of the male 

 gradually degenerate. The male pronucleus then approaches and fuses 

 with the female pronucleus, thus producing a zygote. By two typical 

 meiotic divisions, each accompanied by cytoplasmic divisions, hap- 

 loid agamonts are produced. 



There are a good many variations from the typical series of 

 events just described. A small percentage of individuals that remain 

 in the roach do not encyst or go through any sexual development 

 whatever. During gametogenesis the two resulting cells may be in- 

 completely sexed, becoming gynandromorphs. Such cells attempt fer- 

 tilization but usually fail to complete it. In a few cases two male 

 gametes may succeed in penetrating a female gamete, and their nuclei 



