26 MYCETOZOA AND RELATED ORGANISMS 



has been shown by Howard (1931) that the cell walls are secreted before 

 the spores round up, so that at first a continuous mass of polyhedral cells 

 is produced. The cells then begin to round up and the cell walls split, 

 thus forming the spores. The walls of some species are said to contain 

 cellulose, but some investigators deny this. C. van Wissehngh (1898) 

 claimed the presence of cellulose in Didymium squamulosum (A. & S.) Fr., 

 but denied its presence in Fuligo septica (L.) Gmel. F. von Wettstein (1921) 

 studied the composition of the spore wall in seven other genera and could 

 not demonstrate the presence of cellulose except, doubtfully, in Stemonitis 

 and Reticularia. Neither author found chitin in any of these. The spores 

 are most often violet, purple, or brown. (Fig. 2.) 



By the rupture or dissolution of the peridium the spores are permitted 

 to escape, the capillitial threads preventing all the spores from escaping 

 at once. The fructifications may be sessile or stalked, round or elongated, 

 scattered or crowded, almost microscopic or, in the case of some of the 

 compound fructifications, up to 10 cm. in length and 4-5 cm. in width 

 and thickness. 



In the genus Ceratiomyxa, the only genus of the Suborder Exosporeae, 

 the spores are extruded externally from the fructification instead of being 

 produced internally. The studies of H. C. Gilbert (1935) seem to indicate 

 that this genus may be looked upon as an extreme modification of the 

 condition in Suborder Endosporeae. As in that suborder the sexual fusion 

 occurs by the union of two or more swarm spores followed after some time 

 by the union of pairs of nuclei. The zygotes then enter decaying wood 

 within which they feed, perhaps upon the mycelium of the fungi causing 

 the decay. Eventually the plasmodium creeps out of the wood in which 

 it has been growing and secretes a massive central core of mucilaginous 

 nature, on the outside of which the protoplasm creeps as a thin, some- 

 what reticulate, sheet. In this sheet a further division of the nuclei is 

 followed by a cleavage of the protoplasmic layer into uninucleate naked 

 cells, flattened at the contact surfaces. Each of these secretes a stalk of 

 material similar to that of the main central core, on whose tip the thin- 

 walled spore takes its position. Within this the nucleus divides meiotically 

 to form four haploid nuclei, the spore falling off before or after this stage. 

 It cleaves into four cells in each of which mitotic nuclear division and 

 cleavage occur, so that 8 naked flagellate cells arise from each spore. 

 Gilbert homologizes the stalk upon which the spore sits and the spore 

 with the stalk and sporangium respectively in the Endosporeae. Olive 

 (1907) disagrees with Gilbert, claiming that the nuclear fusions do not 

 occur until just before the formation of the stalks for the external spores. 

 Jahn (1936) agrees with Gilbert that sexual union occurs by the fusion of 

 the swarm cells and their nuclei but locates the meiotic division of the 

 nuclei, as Olive did, in the plasmodium before the stalked spores are pro- 



