INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 437 



suggests that influences arising from the contact and adhesion between the 

 walls of the individual cells must be involved in addition to the acrasin 

 gradients. 



It is worth pointing out that the whole sequence of aggregation, move- 

 ment of the aggregated mass, and eventual formation of the fruiting 

 body, exhibits, as all organic development does, the formation of a 

 definite pattern of differentiated organs as well as mere movement. 



Twitty (1949, Twitty and Niu 1954, Fhckinger 1952) has devoted 

 considerable attention to the factors controlling the migration of the 

 pigment-forming cells from the neural crest in Amphibia. He has shown 

 that one of the main factors is a tendency for the cells to move away 

 from each other. This tendency is increased when the cells are in a closely 

 confined space, which makes it probable that the underlying cause is a 

 movement away from concentrations of waste products. This is the re- 

 verse of what we see in the slime moulds, in which in the aggregation 

 phase the amoebae tend to move together. However, the pigment cells ^ 

 in different species of newt exhibit rather different properties in this 

 respect. Whereas those of Tritums rivularis merely tend to disperse and to 

 remain dispersed, those of T. torosus, after first dispersing, then tend to 

 move back together again into clumps. The location in which these 

 clumps form is influenced by the underlying mesodermal structures, and 

 again there are differences between the species in these mesodermal 

 influences. For instance, some factor in the torosus mesoderm prevents the 

 migration of the pigment cells beyond the dorsal margin of the yolk mass, 

 but this impediment is not offered by the rivularis mesoderm. Its nature is 

 still unknown. Twitty claims that there is little evidence that oriented 

 fibrillar structures in the ground substance, such as those postulated by 

 Weiss, play any part in controUing these pigment cell migrations. 



Abercrombie and Heayman (1952, 1953) have also studied the move- 

 ments of isolated cells, in this case chick fibroblasts in tissue culture, 

 and have emphasised the fact that contact between cells often brings 

 their migration to a halt, these cells apparently having a strong tendency 

 not to creep over one another. 



{b) Movements of tissues: amphibian gastrulation 



Probably the most fully studied instance of morphogenesis and pattern 

 formation by a tissue is the gastrulation and development of the embry- 

 onic axis in Amphibia. The morphogenetic aspect of this comprises the 

 tissue movements of gastrulation, the rolling up of the neural plate into a 

 neural groove and tube, and the subdivision of the sheet of mesoderm into 

 a notochord with rows of somites on each side of it. The pattern formation 



