274 THE BIOLOGY OF BIRDS 



feet. But its shape is such that it does not readily roll, it 

 rotates on its minor axis within a very limited radius. In 

 this case the shape may be secondarily justified by saving the 

 egg from tumbling into the sea. A guillemot that varied in 

 the direction of a spherical egg would not be likely to start 

 a new species ! But it must be kept in mind that auks 

 are affiliated to plovers, so that the top-like shape was 

 probably a legacy from the ancestral stock. 



Shapes of Eggs. — The factors which determine the shapes 

 of the eggs of birds have been discussed by Professor D'Arcy 

 W. Thompson (1908), who points out that in dealing with 

 organic forms, we should try to interpret them in terms of 

 " the intrinsic forces of growth acting from within and the 

 forces of tension and pressure that may have acted from 

 without." 



The problem is : given a practically incompressible fluid, 

 contained in a deformable capsule, which is either (a) entirely 

 inextensible, or (b) slightly extensible, and placed in a long 

 elastic tube, the walls of which are radially contractile, to 

 determine the shape under pressure. 



An incompressible fluid contained in an inextensible 

 envelope cannot be deformed without puckering of the 

 envelope taking place, and, as this does not occur, it may be 

 assumed that the envelope is in some measure extensible, or 

 that the whole structure grows under relatively fixed con- 

 ditions — two suppositions which are practically identical 

 with one another in effect. 



At all points the shape is determined by the law of the 

 distribution of radial pressure within the given region of the 

 oviduct, surface friction helping to maintain the egg in 

 position. Professor Thompson then proceeds to explain 

 that if the egg be under pressure from the oviduct, but 

 without any marked component either in a forward or back- 

 ward direction, the egg will be compressed in the middle, 

 and will tend more or less to the form of a cylinder with 

 spherical ends, as hinted at in the eggs of sand-grouse 

 (Pteroclidae). 



When the egg is subject to the peristaltic contraction of 



