CHAPTER I. 



THE STRUCTUKE OF THE HEX'S EGG, AND THE CHANGES 

 WHICH TAKE PLACE UP TO THE BEGINNING OF INCUBATION. 



1. 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 external 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. 



According to Nathusius, Zeitsch. f. Wiss. Zool. Vol. xvili. p. 225 — 270, 

 xix. 322 — 348, XX. 106 — 120, xxi. 330 — 355, the egg-shell of birds consists 

 of an outer thinner and an inner thicker layer. The outer layer varies con- 

 siderably in its consistency in different species. It is soft and pliant in the hen, 

 but in many other birds, as for instance the ostrich, is hard and friable. It is 

 frequently striated both vertically and transversely. Pigment when present is 

 confined to this layer. The inner layer is thicker ; and its internal surface is 

 marked with rounded processes more or less separated from one another, whose 

 blunt extremities are sunk into the shell-membrane. The presence of these pro- 

 cesses must be considered as universal amongst birds. Vertical sections shew 

 that this layer is composed of alternating horizontal laminae of transparent and 

 opaque material, the opaque laminae being composed of exceedingly minute par- 

 ticles of an organic nature imbedded in a matrix impregnated with calcic salts. 



Both layers of the shell are pierced by vertical canals, which are simple in 

 Carinate but ramified in "Ratite birds. These canals open freely on the exterior 

 surface and also on the interior surface in the pits between the blunt processes 

 of the inner layer. It is probable that the outer openings of these canals 

 become closed by the presence of moisture, so that when the shell is wet neither 

 air nor water can pass through it. If the shell is dry, air will penetrate easily ; 

 and if the upper layer with the free ends of the tubes be rubbed off, both water 

 and air will pass through it without difficulty. In eggs with coloured shells 

 the colouring matter frequently passes into the canals. 



