FOOD AND ITS UTILISATION 99 



of swallowing. It is from this region in the embryo that 

 the gill-clefts grow out and the thyroid gland arises as a 

 median ventral diverticulum. The gill-clefts are transitory 

 structures, mere relics of distant ancestry, except the first 

 one, which persists as the Eustachian tube from the ear 

 passage to the back of the mouth. 



The gullet or cesophagus is an elastic tube, with mucus 

 glands lubricating it internally. It may serve to store food, 

 as in cormorants, without there being any special crop. 

 Every grade may be found between a narrow gullet and a 

 capacious one, between the presence of a huge crop and its 

 entire absence. 



The crop is to be regarded as an enlargement of the 

 gullet for storage purposes. It is not digestive except in 

 so far as salivary juice from the mouth may operate. Along 

 with the food there may be water and mucus, and the 

 fermentation may raise the temperature considerably. A 

 thousand grains of oats have been reported from the crop 

 of one wood-pigeon, and sixty acorns from another. In 

 many cases the crop is seen to be over-crowded with fish, 

 flesh, fruit, or foliage, and the bird may be seen working 

 its neck in a way that suggests some discomfort in its 

 repletion. Vultures may have to disgorge part of their 

 booty before taking to flight ; frightened pelicans and 

 some other birds do the same, often on slight provocation. 

 The skua gull utilises this tendency in order to get fishes 

 from other gulls, and we have often seen a cat chase a 

 wounded gull after its meal and secure what had been but 

 recently swallowed. Regurgitation of food into the mouth 

 of the young bird, e.g. in cormorant and petrel, shows how 

 the abnormal may be normalised. 



We have often watched an eagle working at the pieces 

 of bird in its crop, including skulls and other pieces t»f 

 skeleton, and though this is mainly to make itself more 

 comfortable, it points to the regular habit most marked in 

 owls, of separating the useful from the useless and ejecting 

 the latter in pellets. 



Mr. Beebe (191 7, p. 130) describes the remarkable crop 



