SIDE IvlGHTS ON BIRDS 



of inter-mingled lines. How have these manifold 

 markings been brought about? The leply is not 

 altogether clear even m the present day. That 

 there existed some pigment within the birds' or- 

 ganisation was obvious at once, and this was crudely 

 ascribed to secretions of blood and bile, but what 

 the nature of this colouring matter might be and 

 how it came to be laid on in the manner we see, these 

 questions were left for long undecided. Much of 

 our present knowledge is derived from the re- 

 searches of the late Professor Sorby. By means of 

 spectrum-analysis he discovered seven well-marked 

 substances m the colouring matter of eggs which he 

 distinguished by seven formidable names. One of 

 these is a peculiar red-brown, another a fine blue, 

 and so on. 



In regard to what may be described as the 

 painting of the egg, Professor Newton writes : — 



" In the progress of the egg through that part 

 of the oviduct, in which the pigment is laid on, many 

 of them become smeared, blotched, or protracted 

 in some particular direction. The circular spots 

 thus betoken the deposition of the colouring matter 

 while the egg is at rest, the blurred markings show 

 its deposition while the egg is in motion, and this 

 motion would seem often to be at once onward and 

 rotatory, as indicated by the spiral mai kings not 

 uncommonly observable in the eggs of some bixds 

 of prey and others." 



Newton further remarks that this colouring 

 matter appears to be less profuse in young birds, 

 growing richer and more abundant as the bird 

 attains full vigour and declining again with age. 

 He illustrates this by a series of golden eagles' eggs 

 in his own collection, laid by the same bird duiing 

 a period of twelve years, which show all the grada- 

 tions described. 



38 



