EY i). EMBLETON, M.D. 6l 



tissue is formed in which the calcareous matters are entangled, 

 and crystallize in minute round balls. The first layer of the 

 shell thus deposited is always either white or of the colour of 

 the albumen. 



The layers which come after are either white or of some spe- 

 cial ground colour as cream-coloured, blue, green, or of some 

 secondary tint ; and in many eggs that are spotted one or more 

 of these layers may be spotted, in others the surface layer alone 

 is spotted and always with some tint of red. 



The exact nature of the fluids thrown out by the isthmus and 

 the shell-forming part of the oviduct has not as yet been made 

 out, any more than the peculiarities of these parts of the lining 

 membrane which produce them, as Dr. Thomson in his paper 

 has remarked. 



The egg being completed by the addition of the shell, its size 

 and form are at length fixed, it is more or less roughened on the 

 surface, and has to be propelled down the lower part of the ovi- 

 duct. The muscular tunic is here thicker and stronger than 

 elsewhere, and the lining membrane, with its intense vascularity, 

 is not only stretched to the utmost over the egg, but is ground, 

 as it were, between the hard and rough shell on its inner sur- 

 face, and the powerful muscular contracting agents operating on 

 its outer surface. The egg reaches at length the sphincter of 

 the oviduct, through which it must be forced into the common 

 receptacle, the cloaca, and afterwards propelled through the 

 sphincter ani out into the exterior. All these movements re- 

 quire considerable muscular exertion. 



During the short but variable time that the egg remains in 

 the cloaca it receives a varnish of mucus from the walls of that 

 cavity which aids in its final expulsion and in the protection of 

 its surface. 



This coating is easily rubbed off by the finger wetted with 

 water, and the muddy fluid thus obtained is found when examined 

 under the microscope to contain cells of epithelium from the mu- 

 cous membrane of the cloaca, and numerous minute globular or 

 oval bodies, singly or in little strings, which are particles of 

 urate of ammonia, traces of the urine of the bird which happened 



