330 GENERAL ORNITHOLOGY part ii 



its superior position as before. After all the spiral layers of soft 

 white are laid on, a final covering of dense albumen is deposited at 

 the isthmic part of the oviduct. This forms a tough tunic called 

 the membrana putaminis (Lat. putamen, a peel, rind), or " egg-pod " ; 

 it is the final envelope of such a " soft-shelled egg " as a hen drops 

 when deprived of the lime required to enable her to secrete a hard 

 shell. In the uterine dilatation of the oviduct a thick white fluid 

 charged with earthy matter is exuded; this condenses upon the 

 egg-pod and forms the shell. The composition of this earth is 

 chiefly carbonate of lime (common chalk), with some carbonate of 

 magnesia, and phosphates of both of these bases — thus like that of 

 bone as to ingredients, but in very difierent proportions. The shell 

 does not simply overlie the pod in a distinct sheet, but is intimately 

 coherent, the microscopic crystals or other particles of the earthy 

 matter being deposited in the matted fibrous texture of the pod. 

 The connection is most intimate in fresh eggs ; after a while, layers 

 of the pod separate at the butt of the egg, forming the large 

 air-space which every one has noticed in that situation. The 

 shell being very porous, readily admits air. The air-space enlarges 

 during incubation, and the pod becomes more and more distinct 

 from the shell, which latter also increases in porosity and fragility 

 toward "full term." The rough or smooth appearance of an egg- 

 shell, the pores which may be visible to the naked eye, and other 

 physical characters, are due to the impression made upon it by the 

 lining membrane of the "uterus." The superficial deposit of chalk 

 is so heavy, in some cases, as those of cormorants, etc., that it may 

 be scraped ofi' without interfering with the texturally firm shell- 

 substance underlying. All the coloration of egg-shells, which 

 frequently makes them pretty objects, is simply the deposit of pig- 

 ment granules in or upon the shell. Such deposit may be perfectly 

 uniform, as it is in the bluish -green egg of a robin (Turdus migra- 

 iorius), for instance, but it is oftener spotty — either upon a white or 

 a whole-coloured ground. The browns and neutral tints are the 

 usual colours, particularly a bright reddish-brown ; the same, lying 

 in instead of upon the shell, gives the grays, "lilacs," and "laven- 

 ders" so well known. In ptarmigan, the pigment is so heavily 

 deposited that the egg comes out pasty on the surface ; a sign of 

 " fresh paint ! " one must not disregard if he would not spoil the 

 decoration. 



Oviposition. — The energy and rapidity with which the processes 

 involved in the manufacture of so complex a product as a bird's egg 

 is now seen to be are extraordinary. A domestic fowl may lay an 

 egg every day for an indefinite period. It is difficult to say how 

 quickly an egg may ripen in the ovary ; for, during the activity of 

 that organ, several or many are to be found in all stages of im- 



