INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 449 



Anterior-posterior stretching continues also in the internal dorsal 

 material, namely the presumptive notochord. One could explain the 

 whole morphogenesis of the presumptive mesoderm, into a central 

 notochord flanked by rows of somites, in terms of tendencies for cells to 

 move together in particular regions (Fig. 20.19). We might suppose that, 

 as the mesoderm develops, there is at first a very strong tendency for 

 cells near the midline to increase their surfaces of mutual contact and thus 

 to interdigitate and form a narrow coherent rod. If this adhesiveness 

 fell off rapidly from the midline towards the sides, cells shghtly lateral to 



Figure 20.19 



Diagram of cell arrangement in the mesoderm of the newt neurula, show- 

 ing the developing notochord and somites. 



the midline might have such a strong tendency to move towards it that 

 they were pulled out of contact with cells lying further away; and thus 

 the notochord would be isolated as a long rod of tissue separated from 

 the more lateral mesoderm by a gap. To explain the next stage, the 

 formation of the somites, one would have to suppose that a tendency for 

 cells to move closer together begins to appear in more lateral regions, 

 being at first stronger in the anterior and falling off rapidly towards the 

 posterior. Then a transverse split would appear behind the first group of 

 aggregated cells, isolating the first pair of somites; and if the process 

 gradually spread backwards, the whole series of somites would be 

 successively formed. 



