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



ings of the ovum. " As the ovum passes along the oviduct of 

 the parent it receives its coating of albuminous matter, of which 

 layer after layer is thrown out by the vessels of the oviduct. 

 When a sufficient supply has thus been furnished, it appears 

 that fibrinous instead of albuminous matter is poured forth, and 

 this in coagulating forms a very thin layer of fibrous tissue 

 which envelopes the albumen. Layer after layer is gradually 

 added, and at last, by the superposition of these layers, that firm 

 tenacious membrane is formed, which is afterwards found lining 

 the egg-shell. The process is then continued with this variation, 

 that carbonate of lime is also secreted from the blood in a chalky 

 state, and its particles lie in the interstices of the fibrous network, 

 and give it that solidity which is characteristic of the shell. If 

 they be removed by the agency of a weak acid, or if the bird be 

 not sufficiently supplied with lime at the time of laying, the outer 

 membrane has the same consistence as the inner; and either 

 may be separated after prolonged maceration, by dextrous mani- 

 pulation, into a series of layers of a fibrous matting." I am in^ 

 debted to Professor Fleming for an extract from Purkinje's essay 

 on the Development of the Egg previous to Incubation. He 

 describes the membrane of the shell as composed of two layers, 

 the internal consisting of minute interwoven fibres; the outer, 

 he says, presents no peculiar structure. The calcareous matter 

 first appears in the form of polygonal crystals, which afterwards 

 coalesce. 



Sometimes the surface of the shell is smooth and glossy, occa- 

 sionally unpolished but smooth, in other cases rough and granu- 

 lar, and in not a few covered with a superficial layer which may 

 be easily removed. Such differences (as well as those having 

 reference to colour) will of course affect the transmission of caloric 

 to or from the contents of the egg, and these differences are 

 accounted for by facts now to be stated. 



My aim had been originally to ascertain merely the nature 

 and seat of the colouring matter of the shell ; it is freely acknow- 

 ledged that the observations were begun with a preconceived 

 idea that the calcareous part might be the seat of the colour. It 

 was expected therefore that on placing a coloured egg in diluted 

 muriatic acid, a coloured solution might be procured which could 

 then be examined by chemical tests. The egg of the common 

 guillemot (Uria Troile, Temm.) was first tried; its colour is 

 usually bluish green with dusky blotches, sometimes the ground 

 colour is white. The application of an acid soon proved the co- 

 lour to be partly superficial ; shreds of a fine membrane were de- 

 tached ; these on examination with the microscope were found to 

 possess a distinctly cellular structure ; a pale membrane of like 

 appearance could be separated from the surface of the white 



