346 ON THE SHAPES OF EGGS. [-^W- ^8, 



these are supported directly by the neural spines, in striking 

 contrast to the externally similar anal fin, the rays of which are 

 attached to a series of interhtemals." 



Mr. Henry Scherren, F.Z.S., exhibited on behalf of Mr. Walter 

 Burton, F.Z.S., some melanistic and black leopard skins, presented 

 to Mr. Clarkson Williams in Abyssinia, whei'e they were obtained. 

 The melanistic skins showed a deepening of ground-colour, espe- 

 cially along and on each side of the median dorsal line. In the 

 black skins, of which two were shown, traces of markings were 

 visible in certain lights, and the melanism appeared to be due to 

 a deejDening of the ground, not to a multiplication of the spots. 

 Mr. Scherren stated that black skins from the same locality had 

 been received by Mr. Rowland Ward, and, like these, made up 

 into rugs. Mr. Ward, however, had preferred not to exhibit till 

 he could procure skins obtained by a European sportsman. 



Prof. D'Arcy Wentworth Thompson, C.B., F.Z.S., read a 

 paper entitled " On the Shapes of Eggs"*, which contained a 

 discussion of the physical causes to which may be ascribed the 

 various forms assumed by the eggs of Birds and other animals. 

 The egg, prior to the formation of the hard shell, is a fluid body 

 enclosed in a somewhat elastic membrane. Its form tends to be 

 spherical, but is liable to distortion by the peristaltic pressure of 

 the oviduct. From the nature and direction of the peristaltic 

 wave, compression will be likely to occur in the posterior part of 

 the egg, rendering it broader in front than behind ; and it may 

 be shown mathematically that the form of the envelope or surface 

 ■of the egg will correspond to the formula 



^" + t(^. + ^,)=P' 



where p'^ is the normal component of external pressure at a point 

 where r and r' are the radii of curvature, T is the tension of the 

 envelope, and P the internal fluid-pressure. It may further be 

 shown mathematically that, after the egg is formed, its un sym- 

 metrical shape, with a blunt anterior end and a tapering posterior 

 one, is well adapted for its easy transference through the oviduct, 

 blunt end foremost. The yolk, surroimded by its own proper 

 membrane, remains approximately spherical whatever be the form 

 of the egg-shell : because there lies between the two a fluid (viz. 

 the white of the egg) which makes the pressure (/>") upon the 

 yolk practically constant. 



* Published in 'Nature,' vol. 78. p. Ill (1908). 



