g PRINCIPLES OF EMBRYOLOGY 



sharp but k B a useful rough classification and we shall see that in primary 

 morphogenesis there are many factors wUch play a more important part 

 than di6ferences in growth rate.) 



The two aspects of individuation-morphogenesis and pattern torma- 

 tion-are obviously closely coimected with one another. It is hardly to be 

 supposed that any comphcated three-dimensional structure will devdop 

 uiJess the material out of which it is made has a ready developed a 

 pattern of different properties m its various parts. Thus some degree o 

 pattern formation probably always precedes any but the very sm.plest 

 Lorphogenetic processes. Contrariwise it is to be expected that a develop- 

 ing pattern will be influenced by the shape of the area or mass m which 

 it is forming and we shall find examples which demonstrate that ths is 

 the case. It is, however, helpful to use the distmction between pattern 

 formation and morphogenesis as a means of arranging the subjects which 

 require discussion into some sort of order. Moreover there is a certam 

 difference in the kind of processes which must be involved m the two 

 classes of phenomena. Pattern formation can, and frequently does, go on 

 within a mass whose overall shape does not change. It requires the 

 postulation of forces of an essentially chemical or physico-chemical order 

 -diffusion, facilitated synthesis and the like. Morphogenesis, on the other 

 hand involves the actual movement of masses from one spatial position 

 to another, and requires the intervention of physical forces such as those 

 of surface tension, attraction, contraction, expansion etc. 



Pattern formation and morphogenesis are typical examples of field 

 phenomena, since they mvolve processes which are both extended 

 throughout a region of space and which also have a certam unity. As was 

 suggested earher (p. 23) such fields arise from the interaction or a number 

 of different factors each of which is extended throughout the region 

 involved. We cannot expect, therefore, to be able to attribute the forma- 

 tion of a pattern to the action of any one single factor, but must expect 

 always to fmd that several different things are involved m it; and tlie 

 same expectation of a multiplicity of causes rather than a single cause 

 holds goods for morphogenesis. ■ . j-ff ^ 



Although pattern formation and morphogenesis occur m the differen- 

 tiation of all organs and embryos yet there are not very many instances in 

 which they have been closely studied as the main subjects of investigation, 

 and we still know disappointingly little about the nature of the factors 

 involved in them. What we do know suggests that the operative factors 

 are very different in different cases. The shape of a mass of tissue, for m- 

 stance, may in one case be altered by changes m the tension m the surface 

 of the mass, or again by changes m the adhesiveness of the cell membranes, 



