FEEDING OF THE YOUNG. 



191 



comparing; the state of the crop when the bird is not 

 sitting with its appearance during incubation, the 

 difference is very remarkable. In the first case it is 

 thin and membranous; but by the time the young 

 are about to be hatched, the whole, except what lies 

 on the trachea (wind-pipe) becomes thickened and 

 takes on a glandular appearance, having its internal 

 surface very irregular. It is likewise evidently more 

 vascular than in its former state, that it may convey a 

 quantity of blood sufficient for the secretion of this 

 substance, which is to nourish the young brood for 

 some days after they are hatched. 



" Whatever may be the consistence of this sub- 



A. Crop of a Pigeon when it had no young, 



(The figures A and B represent the crops turned inside outwards and dis- 

 tended with spirits.) 



B. Crop of a male Pigeon while the female was breeding. 

 a. Inner surface of the upper oesophagus. 



6. Ditto lower oesophagus, leading from the crop to the 



gizzard. 



c, c. Inner surface of the two bags of the crop. 



The difference consists in the glandular structure on the inner surface of 

 the crop, which is not seen except in the breeding season. m 



