174 Dr. Dickie on the Structure of the Shell of the 



Nevertheless having failed to discover any special receptacle for 

 such a secretion he adopts the first hypothesis, and supposes that 

 the egg in descending the oviduct presses against the papilla? 

 and causes sanguineous exudations, the ferruginous matter of 

 which produces the brown spots on the shell. The forms of 

 these spots he believes to afford proof of the opinion. The 

 ground colour of eggs (whether uniform or spotted) he considers 

 not to be a property of the calcareous matter as originally secreted, 

 but to be subsequently superinduced, as is the case with the spots, 

 the difference being that in the case of the ground tints the che- 

 mical combination of blood with calcareous salts which produces 

 the colour takes place uniformly , and not partially as in the 

 spots. 



It may not be irrelevant to state briefly the facts recorded by 

 histologists regarding coloured deposits in animal tissues. In 

 the coloured races of men and in some portions of the skin of 

 the white race, the colouring matter is usually deposited in some 

 of the deeper-seated epidermic cells. In the substance of the 

 choroid coat of the eye, in the iris, and in some parts of the scle- 

 rotic coat, there exist pigment-cells of irregular form. The epi- 

 thelium cells of the inner surface of the choroid are also filled 

 with colouring matter. This matter, differing somewhat in co- 

 lour and intensity, consists of oval or oblong grains usually of 

 very small size, T q^ ^th of an inch or less, and crowded in the 

 interior of the cells. 



The ground colour of the egg-shell may reside partly in the 

 epithelium and partly in the deeper layers ; in the former case 

 the action of an acid by removing the epithelium scarcely affects 

 the colour, rendering it only rather paler. As an instance may 

 be mentioned the egg of the hedge-sparrow (Accentor modularis, 

 Temm.) ; its most superficial layer or epithelium is readily de- 

 tached on the application of weak acid ; it is of great tenuity and 

 mostly of a granular structure ; the granules in the mass are of 

 a very faint bluish green ; the separation of this layer scarcely 

 affects the general blue colour of the egg. When all the calca- 

 reous matter has been removed the remaining membranes are still 

 blue, the colour residing in the deeper layers ; the same general 

 arrangement will be found in the eggs of the redstart (Sylvia 

 Pkomicurus, Temm.), the wheat-ear (Saxicola (Enanthe, Bechst.), 

 and Turdus mustelinus, Gmelin, in all of which the colour is 

 nearly similar, differing only in its intensity. 



As already mentioned, the spots may have a well- or ill-defined 

 outline ; both kinds are often to be observed upon the same egg, 

 as in Larus glaucus, Temm., Larus Rissa, Mont., Sterna arctica, 

 Temm., and others. 



The egg of the common thrush (Turdus musicus, Linn.) is 



