274 The Nature of Biological Diversity 



their progression in closely timed series till they have reached their 

 final location in the central epihasal area at 12 days. Then the station- 

 ary cells lose their parallel alignment and hecome multipolar (Fig. 5) . 



In seeking for an explanation for these migratory movements, 

 which resemhle the organized and directed migration of a school of 

 fish in a stream or of a colony of termites, we are faced with 

 several questions: Why should differentiated cells engage in such long 

 routes and settle down in areas so far removed from the ones where 

 they first underwent their early differentiation? Which is the major 

 motive force underlying and directing the migrating cell populations? 



Similar questions were raised in connection with other cell move- 

 ments such as the migratory movements of cells in developmental 

 processes, the disaggregation and reaggregation of cell groupments 

 intermingled at random in tissue culture, and the synergistic move- 

 ments which play such a prominent role in the developmental cycle 

 of slime molds. We shall consider hriefly the results of these investiga- 

 tions since the explanations offered in all three instances may have a 

 bearing on our own problem. 



Migratory patterns of chromatophores in Amphibia were investi- 

 gated by Twitty and by Twitty and Niu (13, 14) in vivo and in vitro, 

 with refined and precise techniques. The results of the experiments 

 performed in vitro suggest that the cells move in response to mutual 

 stimulation or repulsion mediated through the action of substances 

 released by the cells. 



Reaggregation patterns of suspensions of freshly dispersed em- 

 bryonic cells in liquid culture medium were the object of detailed 

 analysis by a number of investigators (15, 16, 17, 18, 19). The early 

 hypothesis by Moscona and Moscona (15), that cell exudates might 

 have an orienting influence on the migration and aggregation of disso- 

 ciated cells, found support in more recent investigations by Moscona 

 (20). He observed that cells dispersed in a liquid medium frequently 

 "appear to follow each other in clearly discernible rows." Phase- 

 contrast, time-lapse movies of similar cultures reveal that the cells 

 move within fine strands of highly transparent slimy substance, evi- 

 dently an exudate of cellular origin. The author suggests that the 

 extracellular matrix might act as a cell-integrating system endowed 

 with specific cell-directing activities. 



The aggregative and migratory patterns of slime molds (21, 22, 23) 

 present a striking instance of cell movements under the controlling 

 action of diffusible substances. The mechanism of the movement and 

 the chemical nature of the agent acrasin, produced by some of the 

 cells, lent themselves to a more precise exploration than animal cells. 



