CHAPTER XV 



ON THE SHAPES OF EGGS, AND OF CERTAIN OTHER 

 HOLLOW STRUCTURES 



The eggs of birds and all other hard-shelled eggs, such as those of 

 the tortoise and the crocodile, are simple solids of revolution ; but 

 they differ greatly in form, according to the configuration of the 

 plane curve by the revolution of which the egg is, in a mathematical 

 sense, generated. Some few eggs, such as those of the owl, the 

 penguin, or the tortoise, are spherical or very nearly so; a few more, 

 such as the grebe's, the cormorant's or the pelican's, are approxi- 

 mately ellipsoidal, with symmetrical or nearly symmetrical ends, 

 and somewhat similar are the so-called "cylindrical" eggs of the 

 megapodes and the sand-grouse; the great majority, like the hen's 

 egg, are " ovoid," a little blunter at one end than the other ; and some, 

 by an exaggeration of this lack of antero-posterior symmetry, are 

 blunt at one end but characteristically pointed at the other, as is 

 the case with the eggs of the guillemot and puffin, the sandpiper, 

 plover and curlew. It is an obvious but by no means negligible fact 

 that the egg, while often pointed, is never flattened or discoidal; 

 it is a prolate, but never an oblate, spheroid. Its oval outhne has 

 one maximal and two minimal radii of curvature, one minimum being 

 less than the other. The evolute to a curve often emphasises, even 

 exaggerates, its features ; and the evolutes to a series of eggs (i.e. 

 to their generating curves) are more conspicuously different than 

 the eggs themselves (Fig. 453)*. 



The careful study and collection of birds' eggs would seem to have 

 begun with the Coujit de Marsiglil, the same celebrated naturalist 



* Cf. A. Mallock, On the shapes of birds' eggs. Nature, cxvi, p. 311, 1925. The 

 evolute may be easily if somewhat roughly drawn by erecting perpendiculars on 

 a sufficient nunrber of tangents to the curve. The .evolute then appears as an 

 envelope, the perpendiculars all being tangents to it. 



t De avibus circa aquas Danubii vagantihus et de ipsarum nidis (Vol. v of the 

 Danubius Panonico-Mysicus), Hagae Com. 1726. Count Giuseppi Ginanni, or 

 Zinanni, came soon afterwards with his book Delle uove e dei nidi degli uccelli, 

 Vcnezia, 1737. 



