xii PHYLUM CHORDATA 483 



" yolks " pass down the oviduct, they are invested with the 

 secretions of its various glands ; first with layers of albumen 

 or " white," next with a parchment-like shell-membrane, and 

 lastly with a white calcareous shell. They are laid, two at a 

 time, in a rough nest, an.d are incubated or sat upon by the 

 parents for fourteen days, the temperature being in this way 

 kept at about 40 C. (104 F.). At the end of incubation 

 the young bird is sufficiently developed to break the shell 

 and begin free life. It is covered with fine down, and is fed 

 by the parents with a secretion from the crop, the so-called 

 " pigeon's milk." 



Of recent birds two main divisions are recognised 

 the Ratitae and the Garinatae the former comprising only 

 the emus (Drom&us}, cassowaries (Casuarius), and kiwis 

 (Apferyx), the South American ostriches (Rhea), and the true 

 ostriches (Struthio) ; and the latter including all the rest. 



One of the most characteristic features of birds in 

 general is the covering of feathers peculiar epidermal 

 structures which differ widely in shape and arrangement 

 from their equivalents, the horny scales of reptiles and the 

 hairs of mammals: The arrangement of the feathers 

 follows closely that briefly described as observable in the 

 pigeon, with great diversity in detail. The distribution of 

 the contour feathers in feather tracts or pterylse separated 

 from one another by featherless tracts or apteria is almost 

 universal in the Carinatae, but is not distinguishable in the 

 Ratitae except in the young condition. In many birds 

 each feather has a secondary vein or after-shaft, as it is 

 termed, springing from the main shaft near the umbilicus, 

 and sometimes (Fig. 294) this may be as large as the main 

 shaft itself. A shedding or "moulting" of the feathers 

 takes place at regular intervals, usually annually a new set 

 of feathers growing from the pulps of the old ones. 



