490 MANUAL OF ZOOLOGY SECT. 



rotates during its passage, becomes coiled at either end into 

 a twisted cord, the chalaza (ch}. Next, more fluid albumen 

 (a/fr 1 ) is deposited layer by layer, then a tough, parchment- 

 like shell-membrane (sh. m} and finally a calcareous shell 

 (sh}. The shell-membrane is double, and at the broad end 

 of the egg the two layers are separate and enclose an air- 

 cavity (a). The shell may be white or variously coloured 

 by special pigments : it consists of three layers, and is 

 traversed by vertical pore- canals, which are unbranched in 

 the Carinatae and in Apteryx, branched in the other Ratitae. 



The eggs may be laid on the bare ground or on the rocks 

 by the seashore, as in penguins and auks, or on the ledges 

 on inaccessible cliffs as in the sooty albatross {Diomcdta 

 fuliginosa) ; but as a rule a nest is constructed for their 

 reception by the parent birds. This may be simply a hole 

 in the sand, as in the ostrich ; a mere clearing on the hill- 

 side surrounded by a low wall of earth, as in the wandering 

 albatross (Diomedea exulans) ; or a cylinder with excavated 

 top, built of grass, earth, and manure, as in the mollymawks 

 (Diomedea melanophrys, etc.). It may take the form of a 

 burrow, as in many petrels, kingfishers, and sand-martins, or 

 it may be more or less elaborately built or woven of sticks, 

 moss, leaves, hair, or feathers, showing every stage of con- 

 structive skill from the rude contrivance of sticks of the 

 pigeon and eagle, to the accurately constructed cup- or 

 dome-shaped nests of many familiar Passeres. In the tailor- 

 bird {Orthotomus} it is formed of leaves sewn together, the 

 beak acting as needle : in a Malayan swift ( Collocalia) it is 

 largely built of the secretion of the bird's buccal glands. 



After the egg is laid, the process of development is ar- 

 rested unless the temperature is kept up to about 40 C. 

 (104 F.) : this is usually done by the heat of the body 

 of the parent birds, one or both of which sit upon, or incu- 



