24 THE PHYSIOLOGY OF REPRODUCTION 



With the approach of the breeding season the genital organs 

 grow enormously until the whole oviduct reaches a state of 

 hypertrophic turgescence. Gadow states that in the common 

 fowl the oviduct at the period of rest is only six or seven inches 

 long and scarcely a line wide, but that at the time of egg-laying 

 it becomes more than two feet in length and nearly half an inch 

 in width, thus increasing the volume about fifty times. This 

 remarkable growth occurs annually. Gadow remarks also that 

 the testes of the house-sparrow increase from the size of a 

 mustard-seed to that of a small cherry, and in so doing tem- 

 porarily displace the usual arrangement of the viscera in the 

 body-cavity. 1 



A very large number of birds seasonally migrate, and this 

 habit, as in the case of the migratory fish already referred to, is 

 closely associated with the function of breeding. 2 Jenner 3 

 stated long ago that migration was invariably associated with 

 an increase in size of the ovaries and testes, and that when 

 these begin to shrink, after discharging their functions, the birds 

 take their departure. Thus the ovaries of the cuckoo are stated 

 to be almost atrophied in July. It would seem quite possible 

 that the annual development of the sexual organs is the 

 immediate stimulus which, in the individual, fixes the time for 

 the spring migration, for it is known that in birds passing 

 northward the ovaries and testes are well developed. (But cf. 

 fishes, p. 18.) Thus wading birds, such as the sanderling shot 

 by Mr. Eagle Clarke at Spurn Head, in May, were found by him 

 to have their sexual organs in a very advanced state of growth. 

 These birds were probably on their way to Greenland or Siberia. 



Schafer 4 has suggested that the migratory impulse is deter- 



1 Gadow, Article on "Reproductive Organs," in Newton's Dictionary of 

 Birds, London, 1893-96. Disselhorst also (" Gewichts- und Volumszunahmen 

 der mannlichen Keimdriisen," Anat.Anz., vol. xxxii., 1908) has called attention 

 to the enormous increase in size and weight of the testicles and ovaries in 

 many birds (and also in some Mammals) in the breeding season. Thus, in 

 Fringilla, the testicles may increase three-hundred-fold. 



2 For much of the information given here regarding migration, I am in- 

 debted to Mr. Eagle Clarke. 



3 Jenner, " Some Observations on the Migration of Birds," Phil. Trans., 

 Part I., 1824. 



4 Schafer, "On the Incidence of Daylight as a Determining Factor in 

 Bird Migration," Nature, November 7, 1907. 



