DEVELOPMENT OF A MYXOSPOEIDIAN 357 



diiiiu becoming surrounded by a condensed mass of protoplasm. 

 The propagative cells may multiply for a time by mitosis, but 

 eventually proceed to spore formation. The propagative cell 

 then divides into two cells of unequal size. Two such cell couples 

 l^ecome associated and the two smaller cells form an envelope 

 around the two larger which form the pansporoblast. The pan- 

 sporoblast is thus from the first composed of two cells. Although 

 the cytoplasm of these cells may fuse later, the nuclei remain dis- 

 tinct and divide repeatedly until the pansporoblast consists of 

 twelve nuclei which may or may not be located in distinct cells 

 inclosed within the envelope cells. The nuclei of the envelope 

 cells, the so-called 'residual nuclei,' degenerate and take no ac- 

 tive part in spore formation. The pansporoblast divides into 

 two cell groups of six cells each, two cells in each group are parie- 

 tal cells, two are capsulogenous cells, while the remaining two 

 unite to form a binucleate cell which becomes the sporoplasm. 

 Later, after the spore is fully formed, the two nuclei unite. 

 Presumably in each sporozoite one nucleus is derived from each 

 of the two original propagative cells but there is no direct evidence 

 that this is the case. On the other hand, Mercier ('10) holds 

 that in Myxobolus pfeifferi the pansporoblast is a zygote formed 

 by the conjugation of a micro- and macrogamete. The single 

 nucleus of the zygote then divides several times to form fourteen 

 nuclei. Twelve of these separate into two groups, the sporo- 

 blasts, each containing six nuclei. Each sporoblast eventually 

 forms a spore while the remaining two nuclei are thrown out and 

 degenerate. 



The principal discrepancy between the accounts of Amerinzew 

 on the one hand, and Schroder and Keysselitz on the other is 

 in regard to the time at which the gametes unite. According to 

 Amerinzew copulation occurs previous to the formation of the 

 sporoblasts while Keysselitz and Schroder hold that it is de- 

 layed until after the spores are formed. It is significant that in 

 Myxobolus pfeifTeri on which Keysselitz worked, Mercier found 

 that copulation takes place previous to the formation of the pan- 

 sporoblasts. This would bring the sporulation of the Poly- 

 sporea in essential agreement with that of the Disporea. 



