BIRDS 



151 



different degrees of curvature, the former almost resembling a cone 

 with its apex rounded off instead of being pointed. Within the 

 outer part of the sclerotic, or firm white external eye -coating, 

 there is a circlet of small overlapping bony plates. The remain- 

 ing most striking difference is seen in the presence of a pecten, 

 a folded structure which projects into the vitreous humour close 

 to the point where the optic nerve pierces the eyeball to branch 

 up in the retina or sensitive innermost eye-coating. 



It was noticed under external characters that there is a third 

 eyelid in addition to the usual upper and lower ones. Some 

 mammals, as the rabbit, possess such an eyelid, though it is 

 not so mobile. 



Development {fig. 104) — All birds lay eggs, protected externally 

 by a firm limy shell, and the lowest mammals agree with them 



Fig. 104.— Structure of Hen's Egg 



in this respect. One or both parents in turn sit upon or incubate 

 the eggs, thus providing the necessary warmth without which the 

 young bird cannot develop. Most of the &gg consists of material 

 which will serve for the nourishment of the embryo, the body 

 of which is evolved from a very small patch {germinal disc), to be 

 seen on one side of the yellow part known as the yolk. The 

 part of the yolk on which the germinal disc rests is lighter than 

 the rest, and this side therefore floats upwards in the glairy matter 

 termed the "white", or more properly speaking the albumen. 

 Hence the embryonic part of the &gg is kept constantly turned 

 towards the source of heat, i.e. the incubating parent. Young 

 pigeons are hatched in a very imperfect state, and have to be fed 

 for some time by the parents, part of their food consisting of a 

 whitish material, the so-called "pigeon's milk", secreted by the 



