654 ON THE SHAPES OF EGGS [ch. 



advantageous to the species in the circumstances under which 

 the egg is laid ; the pointed egg is less apt than a spherical one to 

 roll off the narrow ledge of rock on which this bird is said to lay 

 its sohtary egg, and the more pointed the egg, so much the fitter 

 and UkeUer is it to survive. The fact that the plover or the 

 sandpiper, breeding in very different situations, lay eggs that are 

 also conical, ehcits another explanation, to the effect that here 

 the conical form permits the many large eggs to be packed closely 

 under the mother bird * . Whatever truth there be in these apparent 

 adaptations to existing circumstances, it is only by a very hasty 

 logic that we can accept them as a vera causa, or adequate 

 explanation of the facts ; and it is obvious that, in the bird's egg, 

 we have an admirable case for the direct investigation of the 

 mechanical or physical significance of its formf. 



Of all the man}^ naturahsts of the eighteenth and nineteenth 

 centuries who wrote on the subject of eggs, one alone (so far as 

 I am aware) ascribed the form of the egg to direct mechanical 

 causes. Glinther J, in 1772, declared that the more or less rounded 

 or pointed form of the egg is a mechanical consequence of the 

 pressure of the oviduct at a time when the shell is yet unformed 

 or unsohdified ; and that accordingly, to explain the round egg of 

 the owl or the kingfisher, we have only to admit that the oviduct 

 of these birds is somewhat larger than that of most others, or 

 less subject to violent contractions. This statement contains, in 

 essence, the whole story of the mechanical conformation of the egg. 



Let us consider, very briefly, the conditions to which the egg 

 is subject in its passage down the oviduct§. 



(1) The "egg," as it enters the oviduct, consists of the yolk 

 onlv, enclosed in its vitelhne membrane. As it passes down the 

 first portion of the oviduct, the white is gradually superadded, 



* Cf. Newton's Dictionary of Birds, 1893, p. 191; Szielasko, Gestalt der 

 Vogeleier, J. f. Ornith. Lm, pp. 273-297, 1905. 



t Jacob Steiner suggested a Cartesian oval, r + mr' = c, as a general formula 

 for all eggs (cf. Fechner, Ber. sacks. Ges., 1849, p. 57); but this formula (which 

 fails in such a case as the guillemot), is purely empirical, and has no mechanical 

 foundation. 



J Giinther, F. C, Sammlung von Nestern und Eyern vertchiedener Vogd, 

 Niirnb. 1772. Cf. also Raymond Pearl. Morphogenetic Activity of the Oviduct, 

 J. Exp. Zool. VI, pp. 339-359, 1909. 



§ The following account is in part reprinted from Nature, June 4, 1908. 



