SEXUAL PHENOMENA IN THE PROTOZOA 229 



4. The union of eggs and spermatozoa. 



The union of large with small cells, which is only facultative in the 

 majority of single Mastigophora, becomes obligatory in the Coccidiida. 

 As in the colonial forms just described, Coccidium (schizont) increases 

 usually by asexual reproduction (schizogony). A large number of mero- 

 zoites are produced, each of which may repeat the cycle (Schaudinn). 

 The asexual increase continues for five days (Schaudinn, 'oo), 

 after which the merozoites give rise to sexually differentiated indi- 

 viduals, some of which are large (macrogametes), others small like 

 spermatozoa (microgametes). The phenomena of fertilization have 

 been carefully and independently worked out by Schaudinn ('96) and 

 Siedlecki ('97) in the forms Adelea ovata and Eimeria Schneideri, and 

 by Siedlecki ('98) in Klossia octopiana (Eberthi)(Fig. 127), and Schau- 

 dinn ('oo) in Coccidium. The formation of the spermatozoids has 

 already been described; 1 attention may be called, however, to the pecul- 

 iar central, residual mass which remains after the microgametes are 

 formed, suggesting the residual blastophore in the spermatogenesis 

 of annelids. The microgametes or spermatozoids, when mature, are 

 mere filaments of chromatin with a minimum of cytoplasm ; they are 

 pointed at each end and may or may not have flagella. They move 

 very rapidly by serpentine undulations or by means of their flagella, 

 until they come in contact with a female cell which is fertilized in the 

 same manner as in the Metazoa. The development of the egg is 

 similar to that of the microgamete, so far as the nucleus is concerned, 

 but up to the present no maturation process has been recorded. Sev- 

 eral microgametes cluster around the macrogamete, as spermatozoa 

 group themselves around the egg (Fig. 128). The nucleus of the 

 female cell moves toward the periphery, and one of the microgametes 

 penetrates the nuclear vesicle, and its chromatin fuses completely 

 with that of the female pronucleus, while the entire nuclear mass 

 now moves back toward the centre of the cell. At the same time a 

 peripheral portion of the cytoplasm becomes more dense, refractile, 

 and finally thick and resisting, to form the membrane of the fertilized 

 macrogamete, thus corresponding exactly with the vitelline mem- 

 brane of a fertilized egg. After a thorough mixture of the chromatin 

 in the cleavage nucleus, the latter divides by a peculiar method of 

 mitosis, and a great number of spore-nuclei are formed. 



In Adelea ovata fertilization is effected by the entrance of a male 

 cell through a special opening, the micropyle, while the process is 

 further complicated by the preparatory divisions which the nucleus of 

 the microgamete undergoes before entrance. According to Schaudinn 

 ('97) it divides twice while in contact with the macrogamete mem- 



1 See p. 159. 



