XIII 



PHYLUM CHORDATA 



423 



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 constructive skill, 

 from the rude contrivance of sticks of the Pigeon and Eagle to the 

 accurately constructed cap- 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. 



The number of eggs laid varies from 15-18 in the Partridge to 

 a single one in many Sea-birds and in the Kiwi. As a rule the size 

 of the eggs bears some proportion to that of the Bird, the smallest 



alb 



alb 



Fio. 1085. Callus bankiva (domestic Fow). Semi-diagrammatic view of the egg at the 

 time of laying, a. air-space ; alb. dense layer of albumen ; alb', more fluid albumen ; W. 

 blastoderm ; eh. chalaza ; sh. shell ; sh. m. shell-membrane ; sh. m. 1, sh. m. 2, its two 

 layers separated to enclose air-cavity. (From Marshall's Embryology, slightly altered.) 



being those of Humming-birds, the largest those of the Moas and 

 of ^Epyornis : but in Apteryx the egg is of disproportionate size- 

 as large as a Swan's or an Albatross's, the Kiwi itself being no larger 

 than a barndoor Fowl. 



Segmentation takes place during the passage of the egg down the 

 oviduct, and results, as in Reptiles, in the formation of a blasto- 

 derm (Fig. 1085, bl.) occupying a small area on the upper pole of 

 the yolk. After the egg is laid, the process of development is 

 arrested unless the temperature is kept up to about 37 to 40 C. : 

 this is usually done by the heat of the body of the parent Birds, 

 one or both of which sit upon, or incubate, the eggs until the young 

 are hatched ; but in the Australian mound-makers (Mega- 

 podius) the eggs are buried in heaps of decaying vegetable matter, 

 the decomposition of which generates the necessary heat. 



D D 2 



