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PRINCIPLES OF EMBRYOLOGY 



the direction of migration of any mobile cells present. Such processes 

 almost certainly play a very important part in events such as the out- 

 growth of nerve fibres and the migration of the neural crest cells in 

 vertebrate embryos. They may also be involved in some of the morpho- 

 genetic behaviour of rather more closely packed aggregations of cells. 

 They are one of the mechanisms one might look to in attempting to 

 understand the origin of the five-rayed pattern in the limb-bud, which 

 was discussed above without reaching any conclusion as to the funda- 

 mental mechanism involved. On the other hand, this mechanism can 

 scarcely be held to account for the movements and behaviour of tissues 

 such as those involved in the gastrulation of the Amphibia or the rolling 

 up of the neural plate. 



Figure 20.7 



Above, diagram of the orientation of a reticular matrix between two 



centres of contraction. Below, diagram of the effect of stretching (between 



the arrows) on the shape of mesenchymal cells growing in a reticular matrix. 



(After Weiss 1949.) 



Another investigation on the movement of isolated cells which may 

 have a considerable bearing on morphogenetic processes in general is 

 centred around the rather peculiar situation in the amoeboid slime moulds 

 [Acrasiales). During one stage in their life-history these organisms exist 

 as isolated amoeboid cells. If these are cultured on a surface of nutritive 

 agar, they at first move about in an uncoordinated and haphazard way. 

 Growth and cell division continues until the density has reached some 



