356 H. S. DAVIS 



one nucleus passing into each of the two polar capsule cells while 

 the other two nuclei remain in the sporoplasm. 



In recent years a number of detailed studies of spore forma- 

 tion have appeared, notably those by Amerinzew ('09 and '11), 

 KeysseUtz ('08) and Schroder ('07 and '10). All these writers 

 agree that six cells (eight in Chloromyxum) take part in the for- 

 mation of the spore instead of four as held by earlier writers. 

 Of these six, two form the valves of the spore membrane, two 

 form the capsulogenous cells in which the polar capsules de- 

 velop, and the remaining two the sporoplasm. With this con- 

 clusion my results are in perfect accord. There are, however, 

 important differences in the history of the nuclei during sporula- 

 tion. According to Amerinzew ('09) in Ceratomyxa drepanop- 

 settae the trophozoite has at first two nuclei which are derived 

 from the division of a single nucleus. Each nucleus then di- 

 vides by mitosis into two nuclei of unequal size. The two smaller 

 nuclei are vegetative, the two larger generative. The proto- 

 plasm becomes concentrated around each of the two generative 

 nuclei to form cells inclosed in the endoplasm of the tropho- 

 zoite. These two cells differ somewhat in size, the larger be- 

 ing a macrogametocyte, the smaller a microgametocyte. Each 

 gametocyte then divides to form two gametes. Each gamete 

 extrudes a quantity of chromatin into the cytoplasm after which 

 each microgamete unites with a macrogamete, the single celled 

 zygotes thus formed being the sporoblast from which the spores 

 are developed later. In the formation of the spores each sporo- 

 blast divides into two cells differing in size. The smaller then 

 divides again, giving a sporoblast with three cells, a larger which 

 ultimately gives rise to two 'parietal cells' which form the valves 

 of the spore membrane, and two smaller cells, one of which divides 

 to form the two capsulogenous cells. The nucleus of the other 

 small cell divides to form the bi-nucleate sporoplasm. 



According to Schroder ('07) in Sphaeromyxa sabrazesi and 

 Keysselitz ('08) in Myxobolus pfeifferi the conjugation of the 

 gametes during sporulation takes place at a quite different 

 stage. According to these writers the spores arise from 'propa- 

 gative cells' which originate by generative nuclei of the plasmo- 



