228 The Phenomena of Morphogenesis 



proportions, and cellular structure of the larger ones, surely a remarkable 

 example of the inherent formativeness of these cells. Here form deter- 

 mination finds one of its simplest expressions. The behavior of the myxa- 

 moebae in the Acrasiaceae reminds one of the well-known instance among 

 the sponges where the entire body may be separated into its constituent 

 cells and these later will come together and re-form the body of the 

 sponge. The mechanism by which such morphogenetic movements occur 

 in the slime molds and sponges presents some of the most baffling prob- 

 lems in biology. 



The Acrasiaceae have been studied intensively in recent years. For more 

 detailed accounts of experimental work on them the reader is referred to 

 the publications of Bonner, Raper, Shaffer, Sussman, and Wilson, some 

 of which are listed in the bibliography. The field has been reviewed by 

 J. T. Bonner (1959). 



A somewhat similar example of the differentiation of a formed struc- 

 ture by a mass of undifferentiated cells is shown by a specialized family 

 of bacteria, the Myxobacteriaceae. This is a group in which the individual 

 is a rod-like cell which divides by transverse fission, it lives on other 

 bacteria. In the vegetative period these cells may be distributed through- 

 out a colonial mass or may occur in radiating strands or ridges. Myxo- 

 bacteria do not possess flagella, but their gliding or creeping movements 

 are in some way associated with the abundant slime that they secrete. In 

 the fruiting phase the individuals aggregate into masses, probably under 

 the chemotactic influence of an acrasin-like substance, but no true pseudo- 

 plasmodium is formed. In simple types the aggregates are merely rounded 

 mounds, but in forms like Chondromyces crocatus complex stalked and 

 branching systems are formed. Here, as the rods move upward, piling on 

 top of one another, the mass is constricted at the base and the layer of 

 slime secreted by the advancing rods hardens into a stalk. The apical 

 mass of cells continues to move upward and divides to form branches 

 which culminate in multicellular cysts. In the production of these com- 

 plex fruiting structures by the migration of individual cells, and in the 

 specific character which these structures display, the Myxobacteriaceae 

 resemble the Acrasiaceae, though the groups are not closely related. The 

 same morphogenetic problem as to how a specifically formed structure is 

 produced by independent and undifferentiated cells is presented by both 

 groups of plants. 



A general account of the Myxobacteriaceae has been written by Quin- 

 lan and Raper for Volume XV of the "Encyclopedia of Plant Physiology." 



Another case resembling these but involving much larger size and a 

 higher level of organization is to be found in the development of the fruit- 

 ing body in the fleshy fungi, such as the common mushroom, Agaricus 

 campestris. The vegetative body here is a much branched mycelium which 



