XV] AND OTHER HOLLOW STRUCTURES 659 



peristaltic wave which causes the movement, as well as by 

 friction. 



The quantity T is the tension of the enclosing capsule— the 

 surrounding membrane. If T be constant or symmetrical about 

 the axis of the body, the body is symmetrical. But the abnormal 

 eggs that a hen sometimes lays, cylindrical, annulated, or quite 

 irregular, are due to local weakening of the membrane, in other 

 words, to asymmetry of T. Not only asymmetry of T , but also 

 asymmetry of p^, will render the body subject to deformation, 

 and this factor, the unknown but regularly varying, largely 

 radial, pressure applied by successive annuli of the oviduct, is the 

 essential cause of the form, and variations of form, of the egg. 

 In fact, in so far as the postulates correspond near enough to 

 actuaUties, the above equation is the equation of all eggs in the 

 universe. At least this is so if we generahse it in the form 

 fn + Tjr + T' jr' = P in recognition of a possible difference between 

 the principal tensions. 



In the case of the spherical egg it is obvious that j9„ is every- 

 where equal. The simplest case is where p^ = 0, in other words, 

 where the egg is so small as practically to escape deforming 

 pressure from the tube. But we may also conceive the tube to 

 be so thin-walled and extensible as to press with practically 

 equal force upon all parts of the contained sphere. If while our 

 egg be in process of conformation the envelope be free at any 

 part from external pressure (that is to say, if p^ = 0), then it is 

 obvious that that part (if of circular section) will be a portion of 

 a sphere. This is not unlikely to be the case actually or approxi- 

 mately at one or both poles of the egg, and is evidently the case 

 over a considerable portion of the anterior end of the plover's 



egg- 

 In the case of the conical egg with spherical ends, as is nfore 

 or less the case in the plover's and the guillemot's, then at either 

 end of the egg r and r' are identical, and they are greater at the 

 blunt anterior end than at the other. If we may assume that jt?„ 

 vanishes at the poles of the egg, then it is plain that T varies in 

 the neighbourhood of these poles, and, further, that the tension 

 T is greatest at and near the small end of the egg. It is here, 

 in short, that the egg is most likely to be irregularly distorted or 



42—2 



