INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 45I 



19480). But are they operative among the cells of the gastrula? In most 

 phases of gastrulation and neurulation we still have not enough detailed 

 knowledge of the shapes of the cells adequately to judge the plausibiUty 

 of this. One process has, however, been examined in some detail from 

 this point of view, namely the formation of the notochord in the midHne 

 of the invaginated mesoderm (Mookerjee, Deuchar and Waddington 

 1953)- h is quite clear here that the area of contact between cells does 

 increase considerably as the notochord forms. In fact the cells of the noto- 

 chord soon take up an arrangement which has been compared to a pile 

 of coins, each cell being a more or less flat disc with the maximum possible 

 contact with other similar cells. The later stages of differentiation of the 

 notochord, in which the cells become vacuolated until they are mere dis- 

 tended bags full of cell sap, might be regarded as a consequence of the 

 transformation of more of the original cytoplasm into membrane, so as 

 to increase still more the area of intercellular adhesion (Fig. 20.20). In 

 this example the increase in cell contact is clearly one of the basic pheno- 

 mena of morphogenesis, and it seems not unreasonable to accept it as a 

 causal explanation of the events. Forces arising from the cell membranes 

 may well be the prime cause of the changes in tissue configuration during 

 the whole process of gastrulation and neurulation (Fig. 20.21). 



Figure 20.21 

 Semi-diagrammatic transverse sections of the neural plate of the newt. In 

 A the edges of the plate are only just beginning to fold upwards; the central 

 part of the plate is occupied by nearly equidimensional cells, while at the 

 edge the cells have become columnar and greatly increased the area of cell- 

 to-cell contact. B shows the centre of the plate at a rather later stage, when 

 cells have all become elongated. {B from Lehmann 1945.) 



It must not be supposed, however, that cell-membrane forces are always 

 the most important factors in causing morphogenetic change. Elaborate 

 and definite shapes may indeed be assumed by single cells, as for instance 

 in nematodes where the greater part of the excretory canal forms within 

 the body of one cell. 



There is still one aspect of gastrulation movements which remains to 



