EMBRYOLOGY 197 



arrangement, as may be seen in hard-boiled eggs. Some of the 

 layers of denser albumen, surrounding the fluid layer next the 

 vitelline membrane, extend as twisted cords or " chalazse " towards 

 the two poles of the egg. They do not quite reach the outer layer 

 of the white, although the cord next the pointed pole of the egg 

 ultimately becomes somewhat superficially attached to the lining 

 membrane of the eggshell. The chalazse serve to suspend the yolk 

 by acting as elastic pads, and thus keeping it in position. The 

 interior of each cord presents the appearance of a succession of 

 opaque white knots, hence the name of chalazm or hailstones. 



When the egg has arrived at the narrow consti-iction of the 

 oviduct (which seems to take place in the common Fowl in from four 

 to six hours after its entrance into the infundibular upper end of 

 the oviduct), the mucous membrane of the latter produces a denser 

 layer of albumen mixed with several laminae of felted fibres, which 

 approach the nature of connective tissue. This is the shell -mem- 

 brane which gives the egg its final size and shape, and consists of an 

 inner and an outer layer, both of which remain permanently in 

 close apposition over the greater part of the egg, and adhere to the 

 shell, but at the broad end they tend to separate, and develop an 

 air-chamber between them. This chamber does not exist in per- 

 fectly fresh eggs, but makes its appearance and increases in size as 

 the white of the egg loses in bulk from evaporation. 



From the narrow isthmus the egg passes into the uterine or 

 shell-forming dilatation ; here it remains from twelve to twenty 

 hours. The whole shell is deposited as an accessory sheath by the 

 thickish white excretions of the glandular walls of the uterus. This 

 excretion forms an organic basis or matrix, impregnated with cal- 

 careous mattei% which coagulates and crystallizes partly in the shape 

 of felted strands. The shell rests with so-called mammillary processes 

 upon and partly in the shell-membrane ; the mammillie themselves 

 are comparatively poor in inorganic matter. The interstices be- 

 tween them and the shell-membrane are continued through the 

 calcareous layer of the hard shell as vertical canals. These canals 

 are branched only in the Ostrich, and converge towards the bottom 

 of the little pits on the surface of the egg ; in the Rhea only two 

 canals seem to open into each pit ; in all other birds each pit leads 

 only into one vertical canal. Besides this mammillary and the 

 porous layer, the shell of most birds possesses a cuticular layer. 

 This outermost layer is the most variable part of the shell ; it 

 is apparently structureless, either very poor in calcine salts, and 

 in this case smooth and shiny, or considerably infiltrated with 

 calcareous matter, and then exhibiting the well-known chalky and 

 often rough appearance of the eggs of the Ani, Coi"morants, Grebes, 

 and Fk^mingos. Even when well developed, this cuticular layer is 

 always extremely thin. In the Ostrich and in Rhea it is very 



