252 AVBS. 



the male, there are also clitoris-like structures at the same place in 

 the female. 



Development. Birds are, without exception, oviparous (relation 

 to power of flight). The egg is remarkable for the large amount 

 of yolk (distinguishable into white and yellow yolk), and its porous 

 calcareous shell (fig. 659). The development requires a high tem- 

 perature, at least equal to that of the blood. The necessary heat 

 is usually supplied by the bird during incubation. 



Fertilization takes place in the upper region of the oviduct before 

 the secretion of the albumen and of the shell membrane, and is at once 

 followed by the partial (discoidal) segmentation which only implicates 



the clear part of the 



MT G L^t=L^ W yolk (formative yolk) 



around the germinal 

 vesicle the so-called 

 tread of the cock 

 (cicatricula). 



When the egg is 

 laid, the segmenta- 

 tion is already com- 

 pleted, and the 

 cicafericula has de- 

 veloped into the 

 germinal disc or 



FIG 659. Diagrammatic longitudinal section through an un- 7,7/7 rrn 



developed hen's egg (after Allen Thomson). SI, germinal ( 



disc ; G D, yellow yolk ; WD, white yolk ; DM, vitelline bryo, which later pro- 



membrane ; E W, albumen ; Ch, chalazse ; S, shell membrane; or> f c - Trvrri +1, n 



XS, calcareous shell ;Z, air-chamber. J 6C eyOIK, 



developes, as in Rep- 

 tiles,' the characteristic foetal membranes the amnion and allantois 

 (fig. 635). The duration of the embryonic development varies 

 according to the size of the egg and the relative development of the 

 young when hatched. The Bird, when ready to creep out, breaks 

 the blunt end of the shell by means of a sharp tooth placed at the 

 extremity of the upper beak. 



The young when hatched have essentially the organisation of the 

 adult animal, although they may still be far inferior to it in the 

 degree of thek bodily development. While the Gallinacei and the 

 Cursor, and most Grallatores and Natatores have when hatched 

 a complete covering of down, and are so far advanced in 

 development, that they at once follow the mother on land or into 

 water and there seek their own food (praecoces) ; others like the 



