COLOUR OF EGGS. 121 



of the dilute hydrochloric acid on the markings of 

 the eggs of birds ; for, if we apply this acid to the 

 egg of a song-thrush, which has a ground colour of 

 a bright blue, with irregular spots and blotches of 

 black, these blotches remain while the blue portion is 

 dissolved and disappears. The same takes place 

 with respect to the markings on the eggs of the lap- 

 wing, the chaffinch, the yellow-hammer, the butcher- 

 bird, the magpie, and the house-sparrow. 



From the various examinations which we have made 

 of the coloured markings of these and other eggs, 

 they appear to consist of an animal oil, partly soluble 

 in spirits of wine, similar to that which, as M. Odier 

 has shown, forms the colouring matter of the wing- 

 cases (elytra) of beetles* ; but there seems to be 

 some mucilage also present. This view of the matter 

 is further confirmed by the observations of Mr. Knapp, 

 who tells us that the calcareous matter is partly taken 

 up during incubation, the markings upon the eggs 

 remaining little injured, and even to the last being 

 almost as strongly defined as when the eggs are first 

 laidf. 



From these markings being for the most part either 

 exclusively or more numerous at the larger end of the 

 egg. it would appear that the glands that secrete 

 the colouring matter require the stimulus caused by 

 the pressure of the egg to bring them into activity ; 

 and hence, also, we may account for the zone of 

 markings frequently conspicuous on the eggs of the 

 white-throat, the flusher (Lanius Collurio), and 

 many others. We have just witnessed a fact corro- 

 borative of this explanation. A hen canary which we 

 paired with an aberdevine (Carduelis Spinus) in her 

 second laying, (the first having proved abortive,) had 

 two eggs of different sizes, one of the regular size, 



* Mem. Soc. d'Hist. Nat. de Paris, tome i. 

 t Journal of a Naturalist, p. 223, 3d edit. 



M 



