764 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



Moreover, it is difficult to understand how the experience could be 

 gained except by sight, which in many cases is excluded by the dark- 

 ness. In face of these difficulties, many authorities, such as Professor 

 Newton, have been led to believe that birds have, in an unusual degree, 

 " a sense of direction." 



Development of the Chick 



The ovarian ovum of the hen is a large spherical body, consisting 

 chiefly of yolk, but exhibiting at one region a disc of formative proto- 

 plasm with a large nucleus. The ripe ovarian egg is surrounded by a 

 vitelline capsule, mainly due to the follicular theca in which it is formed. 

 There is an innermost non-cellular membrane, then an epithelium, then 

 a connective tissue outer membrane. The ripening of the egg is 

 accompanied by the disappearance of the nuclear membrane, and also 

 by the formation of polar bodies : but the details of the process are 

 obscure. 



Either before it leaves the ovary, or in the upper part of the oviduct, 

 the egg is fertilised by a spermatozoon. During its passage down the 

 oviduct it undergoes two sets of changes. On the one hand it is sur- 

 rounded by various envelopes added to the delicate vitelline membrane 

 with which it is already invested ; on the other hand, segmentation goes 

 on rapidly in the formxative area. 



The fully formed and laid egg is surrounded by a firm porous shell of 

 carbonate of lime, and beneath this there is a double shell membrane, 

 the two layers of which are separated at the broad end of the shell to 

 form an air-chamber. This chamber grows larger as development pro- 

 ceeds, and is of some importance in connection with respiration, as an 

 intermediate region between the embryo and the external medium. 

 Beneath the shell membranes lies the albumen, or " white of egg," 

 which is secreted by the thin-walled region of the oviduct ; in it lie two 

 spirally twisted cords or chalazae, produced by the rotation of the egg in 

 the oviduct. Within the enveloping albumen lies the ovum proper, 

 with its enormous mass of yolk. The yolk is not homogeneous, but 

 consists of two substances, known respectively as white and yellow 

 yolk. The white yolk forms a central flask-shaped mass, and occurs 

 also as thin concentric layers in the yellow yolk. 



The minimum temperature at which a hen's egg will develop nor- 

 mally is 28° C. If the temperature fall below this, development stops. 



In early stages the interruption may last for days without fatal 

 results, though always with a tendency to induce subsequent abnor- 

 malities. Towards the end of incubation more than a day's cooling 

 is usually quite fatal. 



On the upper surface of the yolk, in whatever position the egg be 

 held, lies the segmented blastoderm, whose exact origin we must con- 

 sider more precisely. 



As we have seen, yolk is to be regarded as an inert and passive sub- 

 stance. In the hen's egg there is an increased specialisation along 

 the line indicated by the egg of the frog. For there is a small patch 

 of formative protoplasm at one pole, and a large aggregate of yolk 



