358 H. S. DAVIS 



On the other hand, in S. dimorpha, I have been unable to find 

 any traces of sexual phenomena during sporulation in either 

 the disporous or polysporous forms, although careful search 

 has been made in both forms for evidence of a sexual process 

 at this time. In the disporous form there is certainly nothing 

 like the process described by Amerinzew in Ceratomyxa drep- 

 anopsetta. As previously described there can be little doubt 

 that in S. dimorpha the sporoblasts are formed by the division 

 of the nucleus of a uninucleate trophozoite, one of the daughter 

 nuclei forming the vegetative nucleus, the other forming a 

 'propagative cell' which divides later to form the sporoblasts. 

 Such trophozoites as are shown in figures 17 to 22 can, I be- 

 lieve, be explained in no other way. There is some evidence 

 (which will be considered in detail later) that the original uninu- 

 cleate cell is a zygote. If this be true, copulation must take 

 place at a much earlier stage than in Ceratomyxa. It might 

 be held that the binucleate trophozoite is formed by the fusion 

 of two uninucleate trophozoites, although I have seen no evi- 

 dence that this is the case. But in that event we would be forced 

 to the remai'kable conclusion that the nucleus of one gamete 

 becomes the vegetative nucleus, while the nucleus of the other 

 alone gives rise to the spores. At present I see no way of har- 

 monizing m}^ results with those of Amerinzew in C. drepanop- 

 settae. However, in Myxidium sp. from the gall bladder of 

 Cottus scorpius, according to Amerinzew Cll), a binucleate 

 trophozoite may form sporoblasts directly without any previous 

 sexual phenomena. In fact, judging from the short account 

 which he gives of sporulation in this species, it differs from S. 

 dimorpha only in relatively unimportant details. 



In the polysporous forms sporulation in S. sabrazesi accord- 

 ing to Schroder, and in M, pfeifferi according to Keysselitz, is 

 in several respects quite different from the same process in S. 

 dimorpha. In all three species the 'propagative cells' may mul- 

 tiply by mitosis. The daughter cells in S. dimorpha are, how- 

 ever, always of equal size and the pansporoblast originate from 

 a single 'propagative cell'. There is no evidence of a preliminary 

 fusion of cell couples as in S. sabrazesi and M. pfeifferi. In 



