436 



PRINCIPLES OF EMBRYOLOGY 



centre of high acrasin production, and other amoebae will move towards 

 them. 



The behaviour of the amoeboid stage of the slime mould is important 

 in providing a clear-cut demonstration that cells can attract their like 

 from a distance. It seems quite probable that processes of this kind play a 

 part, along with the orientation of the ground substance, in controlling 

 the migrations of isolated cells during embryonic development. One 

 must remember, however, that even in the slime moulds the existence of 

 external gradients of acrasin concentration is not sufficient to explain the 

 whole range of the phenomena we observe. For instance, the movement 

 of the sausage-shaped lumps of aggregated cells do not seem to be directly 

 dependent on external acrasin gradients. In these movements, and in the 

 processes leading to the formation of the peg-like fruiting body, Bonner 



Figure 20.9 



Stages in the formation of a fruiting body from the aggregated mass of 

 Dictyostelium cells. A is during the migration of the mass. In B and C the 

 mass is settling down, in D and E the cells at the tip of the mass are becoming 

 elongated stalk cells which push down the axis to the substratum and then 

 raise the whole mass into a peg-like structure. (From Bonner 1952.) 



