BIRDS* EGGS 173 



soon as they are released from the shell. The snipe, though 

 a much smaller bird, can afford to have eggs as large as 

 those of a partridge, because it has only four, which always 

 lie with their points almost meeting in the centre. We 

 believe, however, that some account must be taken of the 

 habits of the various birds, and that a highly nutritive 

 sluggish bird, not flying about much at the breeding season, 

 will naturally have relatively larger eggs than a bird of 

 sparser diet and more active habit. 



Birds' eggs show considerable diversity in shape; for, 

 while the majority are oval, many are " pear-shaped," as 

 in guillemot and snipe, many are approximately spherical, 

 as in owls and kingfishers, those of the sand-grouse are 

 obtuse at both ends, and those of grebes biconical. Even 

 within the species or clutch there may be considerable 

 variability, and a collection of abnormal hens' eggs presents 

 a whimsical appearance. There is, however, a norm for 

 each type, both of shape and size, and it is interesting 

 to observe that after abnormalities " regulative changes " 

 bring subsequently laid eggs back to the normal a 

 phenomenon that leads us to the very heart of vitality. 



The problem of the immediate factors that determine 

 the shapes of the eggs of birds has been discussed in a 

 very interesting paper by Professor D'Arcy W. Thompson 

 (Nature, 4th June 1908, pp. 111-113). The egg, consisting 

 of a slightly extensible membrane filled with an incom- 

 pressible fluid, is subject to external pressure from the 

 radially contractile oviduct, and an equation for the shell 

 that is formed can be worked out. It is pointed out that, 

 from the nature and direction of the usual peristaltic wave 

 in the oviduct, 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 



