490 DEVELOPMENT 



cells and only from these cells, but certain mathematical and 

 physical phenomena have been observed in this connection. 



If one postulates, in the first instance, the presence in the egg 

 of regions denser than others, for example, one can imagine as 

 a result unequal growth in various parts. Unequal growth sets 

 up strain ; and strain, as we saw^ in Chap. XVII., influences the 

 external form and internal structure of organs. This can be 

 demonstrated experimentally by building an artificial blastula of 

 little pellets of dough containing different quantities of yeast. The 

 unequal growth of the various pellets sets up mutual strains and 

 produces a considerable folding and distortion of the whole (Roux). 



Both the circulatory systems and the alimentary canal are 

 evolved from tubular structures, and it is suggestive to hnd that 

 certain phenomena of development are mirrored in inorganic tubes. 

 For instance, a thin test-tube very often cracks in a spiral way. 

 The more homogeneous and isotropic be the glass the more even 

 and regular will be the spiral. That is, the track tends to follow 

 the shortest course on the surface of the tube between the point of 

 origin of the crack and a point diametrically opposite — a ring 

 formation. Generally, how^ever, the ring winds into a helicoid 

 form and is continued. This helicoid geodetic is shown in the 

 coil which stiffens the tracheal tubes of an insect and is apparent 

 in the growth of the intestine. Carey plotted the positions of the 

 cells showing mitosis in the intestinal epithelium at various levels. 

 He found that they formed a left-handed helix (5 per cent, right 

 handed) having its base towards the rectum and its apex towards 

 the ileocaecal valve. Further, the mitotic figures were few near 

 the base and increased in number as the apex was approached. 

 From this he inferred that growth was from below upwards and 

 followed a helicoid path. 



One must consider that a definite mechanical action is due to 

 incidence of stress and that similar results under similar con- 

 ditions are good evidence of the imposition of similar forces. The 

 homogeneous hollow glass tube splits spirally, which can only be 

 interpreted as the spiral path being the line of least resistance in 

 a hollow cylinder. We can apply this with justice to the growth 

 of intestinal epithelium. 



A growth tension applied helically must lead to torsion in a 

 structure like the intestine where there are layers of material 

 growdng at different rates, and one could present a very plausible 

 diagram of forces to explain the twisting and looping of the gut. 

 Similarly and inversely, dilatation, e.g. stomach formation, would 

 produce alterations in the direction of the lines of growth leading 

 to alteration in the arrangement, say, of the muscular fibres. 



