BIRDS' EGGS 



although the shell is secreted by the walls of the oviduct, 

 it seems to be in some measure controlled by the life of the 

 giant-cell the ovum within. 



It must be clearly understood that, if we compare a 

 hen's egg with frog's spawn, what we call " the yolk " is 

 equivalent to the dark-coloured egg-cell which lies within 

 the sphere of jelly ; the latter is equivalent to the white of 

 egg ; but the frog's egg has no hint of a shell ! 



The surface of the shell may be glossy or of very fine 

 grain, as in the kingfisher ; or opalescent, as in some 

 woodpeckers ; or burnished, as if glazed, as in the tinamou ; 

 or pitted, as in the emeu ; or greasy, as in ducks ; or 

 rough, with a chalky film, as in the gannet. In a few 

 cases the chalky layer hides a deeper layer with fine 

 colouring. 



But the most striking feature of birds' eggs is their 

 diversity of coloration, which is often strikingly rich and 

 beautiful. Thanks to the spectroscopic work of the late 

 Dr. H. C. Sorby, who touched many departments of Natural 

 Science with a masterly hand, we know of seven well- 

 marked pigments in egg-shells. Six of these are allied 

 chemically to haemoglobin, the pigment of the blood ; the 

 seventh (lichenoxanthin) is common in plants, especially 

 in lichens, and may possibly be due in some curious way to 

 minute fungi in or on the egg-shell. The other pigments are 

 exuded into the oviduct as the eggs pass down, and there 

 is some reason for connecting their production with the 

 bursting of the eggs from the ovary. 



In any case, minute drops of pigment are exuded from 

 the lining of the oviduct ; they adhere to and stain the egg- 

 shell, which is still soft. If the egg be at rest for the moment, 

 the pigment-drops are fixed as circular spots of colour. 

 But as the egg is usually rotating onwards down the 

 oviduct, the spots of pigment tend to become smeared 



