102 THE BIOLOGY OF BIRDS 



almost continuous degeneration and replacement of the 

 internal cells. But the amount of the milk is greatly in- 

 creased during and immediately after incubation. 



It is interesting to inquire whether the transformation 

 and discharge of the cells lining the crop has any direct 

 connection with the often extraordinary packing and 

 distension of the crop with hard food. For that must 

 exert a very unusual influence on the lining epithelium. 

 Animals often exhibit a capacity for " normalising the 

 abnormal " — witness the way in which the glutinous threads 

 which exude from the male stickleback's kidneys are used 

 to bind the leaves of fresh-water weed or fronds of seaweed 

 into a nest. Is there some similar normalising in turning 

 the pigeon's milk to account in the early nutrition of the 

 young ? In any case, it is obvious that we do not know all 

 about pigeon's milk. 



An interesting fact in regard to pigeon's milk has been 

 discovered by MM. P. Champy and P. Colle (1919). There 

 is a correlation between the activity of the crop and the 

 gonads. During incubation the mucus membrane of the 

 crop of both sexes thickens greatly, and continues to show 

 cellular activity for fifteen days after the eggs are hatched. 

 Now there is a reduction in the size of the testes and an 

 absorption of material from the beginning of incubation 

 onwards, and this coincides with the period of the multi- 

 plication of cells in the lining of the crop. Similarly, in 

 the female there is during the brooding period an absorption 

 of numerous young ova in the ovar}^ The investigators 

 suggest that there is something in the way of a nutritive 

 balance, there is an absorption of material in the gonads 

 when there is expenditure of material on the part of the crop. 

 This seems a plausible idea, especially as there is no trace 

 of any hormone, but the matter requires further investiga- 

 tion, especially in view of the fact that the activity of the 

 crop continues after the regression of the gonads has quite 

 stopped. 



The stomach of a bird may be a simple sac, but in most 

 cases it is divisible into an anterior glandular portion (the 



