254 



ANATOMY OF VERTEBRATES. 



132 





(/f^^^^'^Y)^ 



^r/^ffe-^^^^- 



Allen Thomson has given the annexed illustration, fig. 132, of 

 the structure of the lining membrane of the shell and of the 

 proper shell-membrane. a shows the ' lining membrane of the 

 shell ; a, thick matter or felty portion ; b, thin shred of the torn 

 margin, showing the peculiar fibrous tissue of which the various 

 layers are composed ; b, outermost layer of the same, which is 

 incorporated with the shell ; some of the angular corpuscles of 



the shell lying upon the fi- 

 brous substance and firmly 

 united with it. C, small 

 portion of the calcareous 

 shell, which has been steep- 

 ed in dilute hydrochloric 

 acid, showing the remains 

 of opaque calcareous sub- 

 stance in the centre : here 

 and there clear oval cells 

 seen, as at «, «.' ^ 



The colour of the egg- 

 shell depends on pigmental 

 matter secreted by particu- 

 lar follicles of the villous 

 membrane of the * uterus ; ' 

 and either incorporated uni- 

 formly with the outermost 

 layer of the shell, as in the 

 Thrush: or deposited in 

 cells more or less dispersed 

 or aggregated in patches. 

 The shell consists in great 

 part of carbonate of lime, 

 with a little carbonate of 

 magnesia and phosphate of 

 lime and magnesia. 



The appearance to the 

 unaided eye of pores on 

 the surface of the shell is 

 due to the impressions of the villi of the formative membrane : 

 the permeability of the shell by the atmosphere depends on a 

 more minutely porous texture. The first effect of this per- 

 meability is penetration of air between the layers of the lining 

 membrane as the contents of the egg condense by cold and eva- 



' CCCVIIT. p. 63. 





> 



t^rzM 



^m 



Structure of the shell and shell-membrane in the Fowl's 

 egg. cccviu. 



