XV] AND OTHER HOLLOW STRUCTURP:S 657 



only provided that the pressure tends to make the shape more 

 nearly spherical, and then only on the further supposition that 

 the capsule is also not entirely filled as the deformation proceeds. 

 In other words, an incompressible fluid contained in an inexten- 

 sible envelope cannot be deformed without puckering of the 

 envelope taking place. 



Let us next assume, as the conditions by which this result 

 may be avoided, (a) that the envelope is to some extent extensible, 

 or (6) that the whole structure grows under relatively fixed 

 conditions. The two suppositions are practically identical with 

 one another in effect. It is obvious that, on the presumption 

 that the envelope is only moderately extensible, the whole structure 

 can only be distorted to a moderate degree away from the spherical 

 or spheroidal form. 



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

 distribution of radial pressure within the given region of the tube, 

 surface friction helping to maintain the egg in position. If 

 the egg be under pressure from the oviduct, but without any 

 marked component either in a forward or backward direction, 

 the egg will be compressed in the middle, and will tend more or 

 less to the form of a cylinder with spherical ends. The eggs of 

 the grebe, cormorant, or crocodile may be supposed to receive 

 their shape in such circumstances. 



When the egg is subject to the peristaltic contraction of the 

 oviduct during its formation, then from the nature and direction 

 of motion of the peristaltic wave the pressure will be greatest 

 somewhere behind the middle of the egg ; in other words, the tube 

 is converted for the time being into a more conical form, and the 

 simple result follows that the anterior end of the egg becomes the 

 broader and the posterior end the narrower. 



With a given shape and size of body, equihbrium in the tube 

 may be maintained under greater radial pressure towards one end 

 than towards the other. For example, a cyhnder having conical 

 ends, of semi-angles 6 and 6' respectively, remains in equihbrium, 

 apart from friction, if |> cos^^ -= ^ cos'-^', so that at the more 

 tapered end where 6 is small p is small. Therefore the whole 

 structure might assume such a configuration, or grow under such 

 conditions, finally becoming rigid by sohdification of the envelope. 



T. G. 42 



