io6 SPECIAL CONVERGENCE 



to Owen (op. cit ., vol. ii. p. 160), John Hunter 

 "made many interesting observations on the crop 

 of pigeons, which takes on a secreting function 

 during the breeding season, for the purpose of 

 supplying the young pigeons in the callow state 

 with a diet suitable to their tender condition. 

 An abundant secretion of a milky fluid of an 

 ash-grey colour, which coagulates with acids and 

 forms curd, is poured out into the crop and mixed 

 with the macerating grains. This phenomenon 

 is the nearest approach in the class of birds to 

 the characteristic mammary function of a higher 

 class ; and the analogy of the * pigeon's milk ' 

 to the lacteal secretion of the mammalia has 

 not escaped popular notice. . . . The secretion 

 consists of proteine with oil, but contains no 

 sugar of milk nor fluid caseine." 



An analogous secretion has been discovered 

 by Alcock to take place in the uterus of vivi- 

 parous rays. In pregnant females of sting-rays 

 (Trygon\ of the eagle-ray (Myliobatis\ and in 

 the bat-ray (Pteroplataa), Alcock found that the 

 young are nourished before birth by a milky 

 secretion that exudes from glandular filaments 

 or villi on the inner surface of the uterine wall, 

 a large bundle of the filaments passing through 

 each spiracle into the pharynx of the immature 

 fish. The milk-secreting filaments are penetrated 

 by a capillary network in the meshes of which 

 the milk-glands are imbedded. Each filament 



