290 GEORGE W. BARTELMEZ 



yolk, in the case of those oocytes in which such conditions as are 

 shown in figure 7 existed during the later stages of the period of 

 differentiation. There are many oocytes in which the 'typical' 

 conditions hold throughout the entire development. 



D. The final rapid growth period 



The ovary of an adult unmated pigeon has normally fi-om six 

 to twelve follicles in the terminal stages of the period of different- 

 iation, from 3 to 5.5 mm. in long axis; never any larger ones. 

 That is to say, the process of rapid yolk secretion which charac- 

 terizes the final growth period is initiated by the stimulus of mating. 

 The same holds true for birds that have reared young; the stim- 

 ulus is received when sexual activity is resumed after the young 

 leave the nest. The only theory that will account for all the 

 observed facts is that the initial stimulus is psychic in character; 

 the bird may be mated with another female, with a bird in another 

 cage, or even with her caretaker, and yet lay eggs. This state- 

 ment is based on Professor Whitman's extended observations in 

 breeding pigeons, and, needless to say, it is easy to tell when a 

 bird is mated from her behavior. Harper ('04, p. 4 ss.)- 



Corresponding to the wide experience that pigeons never lay 

 more than two eggs at a sitting, one usually finds in an active 

 ovary two follicles, differing slightly in size, which are distinctly 

 larger than the rest. When the stimulus is received these two 

 begin to grow, one always keeping a few millimeters larger than 

 the other. Occasionally, (in 6.2 per cent of a total of 261 cases), 

 it happens that three follicles mature at the same time, one of 

 them larger than the other two; a condition which is considered 

 on p. 294; it should be noted that an arrangement of follicles in 

 sets in the hen has been observed by Patterson ('10, p. 105). 

 In the course of about eight days the oocyte grows from 5 to 20 

 mm. in long axis, the definitive size being relatively constant for 

 the eggs of a given bird. The average is about 20 mm., long 

 axis, 18 mm., polar axis, and 18.5 mm. for the third axis perpen- 

 dicular to these two ; but the eggs obtained from one bird averaged 



' See also W. Craig, Oviposition induced by the male in pigeons. Jour. Morph., 

 vol.22, p. 299, 1911. 



