The Beginnings of Multicellular Organization 33 



level by a mutant of Dictyostelitim discoideum that can form normal 

 fruits containing hundreds of thousands of cells or as Httle as 10-12 cells. 

 Figure 16 shows an example of the latter, a fruiting body with 12 cells— 9 

 spores, 2 stalk cells, and 1 basal disc cell. This exquisite miniature contains 

 all the elements of a morphogenetic system— cellular differentiation, cell 

 interactions, and regulation— but in a tiny packet of cells. 



In summary, these are some of the lessons learned thus far from the 

 study of slime molds: 



a. That just as in the development of higher forms (vertebrate em- 

 bryos, etc. ) , slime-mold development involves the appearance of new cell 

 types (initiator and responder cells; leader and follower cells; spores, stalk 

 and basal disc cells) which play a causal role in the construction of the 

 multicellular whole. We would like to know how and when they arise and 

 in what numbers, and what physiological mechanisms are responsible 

 for the roles they play in morphogenesis. 



b. That here as in other developmental systems, a matrix of cellular 

 interactions helps to regulate the process. Chemical signals are passed 

 between initiator and responder cells, between responder and responder 

 during aggregation, and between leader and follower cells during migra- 

 tion of the slug. Finally, chemical signals tell some cells to become spores, 

 others to become stalk cells, and still others to become basal disc cells in 

 the fruiting body. We want to know what are these signals and how they 

 act. 



c. That normal development need not involve participation of huge 

 numbers of cells. Instead, even as few as 10 cells can provide the com- 

 ponents and the chemical interactions needed to construct a perfect fruit- 

 ing body. 



A Water Mold, Achlya Bisexualis 



Achlija is an example of a permanently colonial organism. The cells 

 grow in long, branched filaments which form a tangled mass. Some of 

 these filaments can penetrate the substratum ( a floating seed, a dead in- 

 sect) to anchor the colony and absorb nutiient materials and thus serve 

 as a primitive root system. Others grow as aerial stalks and produce special 

 structures called sporangia that bear spores. The spores are cast off and 

 germinate to yield new filamentous colonies. Some of the aerial filaments 

 can also produce sexual structures, primitive counterparts of the male and 

 female structures of plants. A single Achlya colony produces either male 

 or female structures, not both. The fascinating aspect of this development 

 is that the formation of male and female structures is triggered by hor- 

 mones that pass between the colonies. 



