POLAR BODIES IN PROTOZOA 



189 



flead 



nuts. 



the reduced number of chromosomes, while the sporophyte (fern 

 plant proper), owing to the fact that it develops from a fertilized 

 ovum and receives a set of chromosomes from each parent, has the 

 full number, or, perhaps we should say more correctly, the double 

 number. 



Even amongst the Protozoa the phenomenon of nuclear reduc- 

 tion has been observed, and it probably occurs wherever the sexual 



process takes place. Thus, 

 in the case of Copromonas, 

 the life history of which we 

 have already dealt with, 

 the nucleus of each of the 

 similar gametes, previous 

 to uniting with its mate to 

 form the zygote nucleus, 

 gives off two "polar bodies" 

 which undergo degenera- 

 tion in the cytoplasm (Fig. 

 37, 3, 4), and something very 

 similar may be observed in 

 the maturation of the ovum 

 of Coccidium. In the con- 

 jugating Paramcecium, 

 again, the products of divi- 

 sion of the micronucleus 

 (Fig. 41, B, C) which undergo 

 no further development may 

 be regarded as polar bodies. 

 As we have already seen, 

 the maturegametes through- 

 out both the animal and 

 vegetable kingdoms usually 

 sexual dimorphism, which 

 into active 



Tail 



FIG. 69. Diagram of typical Sperma- 

 tozoon and Ovum, the former much 

 more highly magnified than the 

 latter. 



ax., axial filament; e.g., chromatin granules; 

 cs., centrosome; cyt., cytoplasm; m.p., 

 middle piece; n.m., nuclear membrane; 

 nil,., nucleus; mils., nncleolus ; v.m., 

 vitelline membrane ; y-g-, yolk granules. 



very strongly marked 



exhibit 



attains its fullest expression in the differentiation 

 spermatozoon and passive ovum (Fig. 69). 



A typical spermatozoon, as we have also pointed out, closely 

 resembles a flagellate protozoon. It consists of a " head " and 

 a "tail," connected together by a " middle piece " (m.p.). The 

 head contains the nucleus (MM.), which almost entirely fills it, 

 being covered with only a very thin envelope of cytoplasm. The 

 middle piece contains a centrosome (cs.). The tail, or flagellum, is 



