CHAPTEE I. 



THE STRUCTURE OF THE HEN'S EGG, AND THE CHANGES 

 WHICH TAKE PLACE UP TO THE BEGINNING OF IN- 

 CUBATION. 



In a hen's egg quite newly laid we meet with the 

 following structures. Most external is the shell (Fig. 

 1, s.), composed of an organic basis, impregnated with 

 calcic salts. It is sufficiently porous to allow of the 

 interchange of gases between its interior and the exter- 

 nal air, and thus the chemical processes of respiration, 

 feeble at first, but gradually increasing in intensity, are 

 carried on during the whole period of incubation. 



It is formed of two layers, both of which may contain 

 pigment. The inner layer is by far the thickest, and is 

 perforated by vertical canals which open freely on its 

 inner aspect. Superficially these canals appear to be 

 closed by the extremely thin outer layer. They are 

 probably of some importance in facilitating the pene- 

 tration of air through the shell. 



Lining the shell, is the shell-membrane, which is 

 double, being made up of two layers : an outer thicker 

 F. & B. 1 



