242 THE PROTOZOA 



types. In the subdivision termed the Sorophora or Acrasiae there is no 

 flagellula-stage in the life-history, and the amcebulae which are produced 

 from the spores aggregate together, but form only a pseudo-plasmodium, 

 in which the constituent amcebulae remain distinct, without fusion of their 

 protoplasmic bodies, each amoebula multiplying independently. The details 

 of the reproductive process also vary greatly. In the division known as 

 the Exosporeae, represented by the genus Ceratiomyxa, no sporangium is 

 formed, but the plasmodium grows up into antler-like processes, sporophores, 

 over the surface of which the plasmodium divides up into a mosaic of cells, 

 each containing a single nucleus of the. plasmodium. Each cell becomes 

 a spore, which is produced on the free surface of the sporophore, and drops 

 off when ripe. In the Sorophora the amoebae associated in the pseudo- 

 plasmodium are not all destined to become reproductive individuals ; some of 

 them join together to; secrete a stalk, and develop no further ; others form 

 clusters (" sori ") of naked spores on the stalk. 



The cytological details of the life-history of the Mycetozoa have been the 

 subject of a series of studies by Jahn, who, however, in his latest investigations, 

 has come to conclusions different from those at which he arrived in his earlier 

 works. According to the earlier accounts given by Jahn and Kranzlin, the 

 spore -formation was preceded by a fusion of nuclei in pairs throughout the 

 sporangium, a process which was regarded as the true sexual karyogamy, 

 and was followed by reducing divisions. According to Jahn's latest investiga- 

 tions (294), however, the nuclear fusions observed in tne sporangium take 

 place only between degenerating nuclei, and are to be interpreted as purely 

 vegetative phenomena which have- nothing to do with the true sexual process, 

 which is stated to be as follows : The nuclear division which immediately 

 precedes spore -formation is a reducing division, whereby the number of 

 chromosomes is reduced from sixteen to eight. Consequently the nuclei of 

 the spores, and also the swarm-spores produced from them, both flagellulae 

 and amcebulae, have half the full number of chromosomes. In Physarum 

 didermoides the amcebulae multiply by fission, with mitoses showing eight 

 chromosomes. After a certain number of such divisions, the amcebulae 

 copulate in pairs as gametes. The zygotes thus formed are the foundation 

 of the plasmodia ; when one zygote meets another it fuses with it, the nuclei 

 remaining separate, and by repeated fusions of this kind the plasmodia are 

 formed. When, on the other hand, a young plasmodium or a zygote meets 

 an amcebula (gamete), it devours and digests it. The nuclei of the plasmodia 

 ^multiply by mitoses which show sixteen chromosomes. 



In Ceratiomyxa the reduction- division preceding spore-formation is followed 

 by degeneration of one of the two daughter- nuclei ; the other becomes the 

 nucleus of the spore. Within the spore the nucleus divides twice, forming 

 four nuclei, and as soon as the spore germinates the contents divide into four 

 amcebulae, which adhere in the form of a tetrahedron. Each amcebula has 

 eight chromosomes in its nucleus, and divides into two amcebulae, also with 

 eight chromosomes. Each of the amcebulae develops a flagellum and swims 

 off. Possibly in this genus the syngamy takes place between flagellulae. 



From the investigations of Jahn, it is clear that the swarm-spores of 

 Mycetozoa, like those of other Sarcodina, are the gametes ; their nuclei have 

 undergone a process of reduction, and represent pronuclei, which after a 

 certain number of divisions give rise by syngamy to synkarya, from which 

 the nuclei of the vegetative phase, the plasmodium, takes origin. 



The Mycetozoa are classified by Lister (297) as follows : 



SUBORDER I. : EUPLASMODIDA (Myxogastres, Myxomycetes sens, strict.). 

 Mycetozoa with a flagellula-stage and a true plasmodium formed by plasto- 

 gamic fusion of amcebulae. This suborder comprises forms with the full life- 

 cycle described above. 



Section 1. Endosporece. Spore -formation within a sporangium. Examples : 

 Badhamia, Fuligo (^Ethalium), etc. 



Section 2. Ectosporece. Spores formed on the exposed surface of sporo- 

 phores. Example : Ceratiomyxa. 



