HIGH SCHOOL ZOOLOGY. 131 



but in the birds, especially in the song-birds and parrots, it is not 

 only of considerable range, but also capable of modulation. The 

 larynx, which is the organ of voice in the other Vertebrates, is 

 here in the background, for the notes of the song-bird are pro- 

 duced lower down in the wind-pii^e, at the point where it divides 

 into the two bronchi. Here the " syrinx " is situated, in the 

 foi-mation of which, both membranes capable of vibration, and 

 resonant dilatations capable of reinforcing the sounds produced 

 by these, take part. 



12. The kidneys in the birds are not so elongated as in the 

 reptiles, and are moulded into the large and complex sacrum. 



Only one oviduct, the left, is present ; the number of eggs laid 

 is very difterent in different species, but approximately constant 

 in the same species. The size is not always directly propor- 

 tionate to the size of the bird, for the chicks escape from tlie 

 egg (by the agency of a temporary tooth on the upper beak) at 

 very different periods of their development. Of the various 

 ordci-s of Carinate birds, those which are the more primitive 

 esca])e from the egg in a condition to fend for themselves (aves 

 precoces), while the young of the higher orders required to bo 

 looked after by the parents for some time after they are hatched, 

 their escape taking place at a much less developed phase (aves 

 altrices). 



13. The egg of the fowl owes its large size chiefly to the food-yolk, 

 which is associated with the germinal yolk (I,GG). The yolk is, uever- 

 theless, a single cell bounded by a wall, the vitelline membraue. After 

 bursting through its capsule iu the ovary (Fig. 94), it escapes into the 

 ca'lom, and is received by the open mouth of the oviduct, the walls of 

 which are provided with glauds, which secrete the albumen or white, 

 and with muscular layers, which propel it iu a spiral direction (involving 

 the formation of the ropy jjarts of the white) to^v•ards the lower end of 

 the tube, where the shell-glands secrete the shell. When the egg is 

 laid, it has already iindcrgone some of the stages of segmentation, the 

 white patch upon the surface being formed of a layer of cells (the 

 blastoderm), destined to grow into the body of the chick. The jirocess 

 of incubation retpiires tweuty-oue days, and it can be carried out 



a> 



