450 



PRINCIPLES OF EMBRYOLOGY 



The tendencies of cells to move together, which have been postulated in 

 the above scheme, are in most cases real enough, as can be seen by examin- 

 ing the embryos at various stages. The hypothetical element is the 

 suggestion that these tendencies for particular types of aggregation are 

 produced by forces arising from the adhesiveness of the cell membranes. 

 Again, we know that such forces exist in isolated cells (cf Holtfreter 1947, 



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Figure 20.20 



Stages in the development of the rounded cells of the archenteron roof into 

 the notochord in the newt. At stage b the cell boundaries are difficult to see; 

 at c the chordal sheath begins to be formed. The whole process of develop- 

 ment consists, in the main, in an increase in the relative extent of the cell 

 surface. (From Mookerjee, Deuchar and Waddington 1953.) 



