INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 439 



membrane and the external cytoplasmic membrane, often show a double 

 refraction due to the orientation of the molecules composing them 

 (Reviewed: Frey-WyssHng 1948). 



In most cells, however, there is httle evidence that the bulk of the 

 internal cytoplasm also contains orientated structures of this kind, al- 

 though during many of the early morphogenetic processes in amphibian 

 embryos some cells assume elongated wedge-like or flask-like shapes, 

 such as might be expected on this hypothesis. This occurs, for instance, in 

 the cells lining the early blastopore, in the neural groove and in ectodermal 

 in-foldings such as the lens. Polarised light cannot be used in these cases, 

 since the cells are still full of yolk granules which are highly refractile and 

 obscure the picture presented by the cytoplasm itself. However, the yolk 

 granules are not spherical, but somewhat ovoid, and if the cytoplasm 

 possessed any strongly oriented fibrillar structure, one might expect 

 that the yolk granules would lie with their long axes parallel to it. In- 

 spection of sectioned material does not reveal any clear evidence of such 

 orientation (Waddington 1942c), and it is therefore probable that intra- 

 cellular fibres play at best a very minor role in amphibian early morpho- 

 genesis (at least, directly, cf. Lawrence et al. 1944). The nuclei, which are 

 of course much larger than the yolk granules, are usually very defmitely 

 orientated in the same direction as the main body of the cells, and the 

 appearances strongly suggest that this is because they have been squeezed 

 into these positions by the constraining cell walls, which would therefore 

 appear to exert an important effect in determining the shape of the whole 

 cell. 



Recent work on the forces producing morphogenetic change in the 

 amphibian embryo has, in fact, been led from several points of view to 

 attach great importance to the behaviour of cell membranes, both those 

 between the cell and the external medium, and between cell and cell. 

 As has been particularly emphasised by Holtfreter (1943 a, 1943-44), the 

 external (cell-medium) membrane of the early amphibian egg has several 

 peculiar properties. Its outer surface is more or less solid and non-adhesive, 

 and it has a great capacity both for elastic expansion and contraction and 

 for plastic flow. As the egg becomes segmented this surface layer keeps 

 fusing up again across the newly appearing cleavage furrows, so that it 

 remains as a continuous undivided sheet (the so-called 'coat') connecting 

 the cleavage cells. It is, indeed, the main thing which holds the cells to- 

 gether. If it is dissolved by treatment with alkaline solutions the cells fall 

 apart, since there is little tendency for the internal cell membranes to 

 adhere to one another at this stage. As well as binding the cells together, 

 the coat has also important osmotic properties, being less permeable to 



