326 BIOLOGY OF THE PROTOZOA 



agative cells later unite 2 by 2 and are at first separated by a thin 

 cell wall, which later disappears. Within this united mass the 

 nuclei divide until there are 14 as in Sphaeromyxa. Such cases of 

 fusion are interpreted by Erdmann (1917) as plastogamous in 

 character. 



A more complicated history is furnished by Naville (1931) for 

 Chhromyxum leydigi. Here the initial organism has a large number 

 of nuclei which divide by mitosis, each nucleus with 4 chromosomes. 

 These are succeeded by heteropolar mitoses with 2 chromosomes 

 at each pole. Two types of nuclei are thus formed, large and small, 

 each with 2 nuclei. Upon internal bud formation a large nucleus 

 unites with a small one. After fusion the fusion nucleus divides 

 several times, each time with 4 chromosomes. Later each undergoes 

 a reducing division and nuclei with 2 chromosomes again result. 

 Each of these haploids divides to form a group of 8 nuclei in the 

 sporogenous plasm and spores are formed as in other species, each 

 spore having 2 haploid nuclei which unite later. 



These observations indicate that fertilization in Myxosporidia 

 belong in the group of autogamous phenomena. In the closely 

 related Microsporidia there is considerable difference of opinion in 

 connection with the time and place of fertilization if it occurs at 

 all. Stempell (1902, 1904, 1909) and Fantham and Porter (1912) 

 give evidence to indicate that union of nuclei occurs as in Myxo- 

 sporidia and after the spore leaves its capsule. Mercier (1909), 

 Swarczewsky (1914) and others believe that the formation of hetero- 

 gametes occurs prior to sporulation as described by Awerinzew 

 for Ceratomyxa; Debaisieux (1913, 1915, 1916) also believes in a 

 process of autogamy prior to sporulation in Glugea danilewskyi, G. 

 mulleri, G. anomala, and in microsporidian parasites of Simulium 

 larvae. 



Similarly a process of autogamy occurs prior to sporulation in 

 Actinomyxida. Here, according to the observations of Caullery 

 and Mesnil (1905) on Sphaeractinomyxon, the youngest stages are 

 found as intestinal parasites of the tubificid worm Clitellio, and are 

 either uninucleated or binucleated. The observers were inclined 

 to believe that the uninucleated stage comes first and that it repre- 

 sents, possibly, a sporozoite. Whatever may be the origin of the 

 binucleated form, the 2 nuclei divide and 2 of the 4 resulting nuclei 

 become somatic nuclei connected w T ith the formation of the cyst 

 wall. The remaining nuclei and cell body now divide until there 

 are 16 independent cells. These unite 2 by 2, fertilization thus 

 occurring endogamously, and 8 spores are finally formed. 



In many of these cases so-called reduction nuclei have been de- 

 scribed as indicating processes comparable with chromosome reduc- 

 tion in meiosis. Up to the present time, however, while well-marked 

 chromosomes of definite number have been described by George- 



