940 ON THE SHAPES OF EGGS [ch. 



little capable of extension and so completely filled as to preclude the 

 possibility of deformation. 



If the capsule be not spherical, but be inextensible, then only such 

 deformation can take place as tends to make the shape more nearly 

 spherical; and as the surface area is thereby decreased, the envelope 

 must either shrink or pucker. In other words, an incompressible 

 fluid contained in an inextensible envelope cannot be deformed 

 without puckering of the envelope. 



But let us next assume, as the condition by which this result 

 may be avoided, that the envelope is to some extent extensible and 

 that deformation is so far permitted. 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 withih 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 some- 

 where 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. 



The peristalsis of the oviduct thus plays a double part, in pro- 

 pelling the' egg down the oviduct and in impressing on it its ovoid 

 form; but the whole process is a very slow one, for the hen's oviduct 

 is only a few inches long, and the egg is some ten or twelve hours 

 upon its way. We shall consider presently certain shells which 

 may be regarded as so many drops or vesicles deformed by gravity; 

 that is a statical problem. Compared with it the problem of the 

 egg is a dynamical one; and yet it becomes a quasi-statical one, 

 because the action is so very slow. It is an action without lag 



