INDIVIDUATION— FORMATION OF PATTERN AND SHAPE 433 



pletely uniform and homogeneous body which it seems, but has certain 

 elements of structure which provide a basis on which the more elaborate 

 patterns of later development may be constructed. Even in examples 

 such as those we have chosen to discuss, of the Drosophila wing and the 

 vertebrate limb-bud, where the earHest stages show no obvious sign of 

 even a trace of inherent pattern, it may well be that this apparent homo- 

 geneity is partly deceptive. Perhaps after all we are never confronted 

 with the origin of pattern from the completely formless but only with 

 increases of complexity of pattern. But even so, the problem of how this 

 occurs is difficult enough and at present almost completely beyond our 

 understanding. 



4. Morphogenesis 



The word 'morphogenesis' is often used in a broad sense to refer to 

 many aspects of development, but when used strictly it should mean the 

 moulding of cells and tissues into definite shapes. Morphogenesis, like 

 pattern formation, is thus a phenomenon which is involved in almost 

 every instance of differentiation. It is clearly impossible to discuss every- 

 thing that is known about it. We shall therefore consider only one or two 

 examples which will serve to illustrate the nature, and the successes and 

 limitations, of some of the approaches which have been made to the 

 problem. 



[a) Movements of isolated cells 



One of the simplest types of morphogenetic process is oriented move- 

 ment by isolated cells. Weiss (1933, 1945) has devoted particular atten-r 

 tion to this as a factor in normal development. By experiments on cells 

 growing in tissue culture, he has shown very clearly that when cells 

 creep about by any form of amoeboid movement a very powerful 

 influence on the direction of movement is exerted by the microstructure 

 of the medium or surface on which the cells are placed. Thus if the cells 

 are provided with a fibrous substratum, such as glass wool, they tend to 

 creep along the fibres. The same is true when the fibres are sub-micro- 

 scopic in scale. If a plasma clot is stretched, the protein micelles of which 

 it is composed become partially oriented, and both the shape and the 

 direction of movement of the cells fits into the ultra-structure of the 

 medium (Fig. 20.7). 



Weiss points out that in a developing embryo the intercellular spaces 

 are filled with a 'ground substance' or protein-containing jelly. The differ- 

 ential growth of different parts will stretch this in certain ways, and by 

 thus orientating the ultra-structure of the ground substance, influence 



