PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 



341 



and axis angle variation so in view of Professor Whitman's 

 results the layings were not forced (see Riddle, 1914). When the 

 eggs were removed dummies were left in their places for about two 

 weeks until the birds recognized the deceit or the futility of further 

 incubation and made preparations for another laying. From time 

 to time each pair was permitted to hatch and rear young. Under 

 these conditions the average number of clutches per year was 16, 

 a figure which has been considerably lowered by inclusion of the 

 records of the old birds at the end of their active laying period. 

 The spring was the most prolific season, the autumn the least so, 

 as Pearl and Surface (1911) observed for the hen. 



The angles were measured as described in 1912. As some 

 pigeon eggs show little difference between the two ends of the 

 shell the position of the air space was always noted; its end of 

 the shell is readily seen when the egg is placed in the salt solution 

 since it floats up higher than the other. The blunt or air space 

 end of the shell was held to the left, the other end removed with 

 forceps and the contents drawn out. Thus any possible con- 

 fusion between the two chalazse was avoided. Except during 

 the first year the eggs were left under the birds for two to three 

 and one half days. As pigeons are usually very lax about 

 incubating until the second egg has been laid the embryos varied 

 from the five to the twenty somite stages. During this period 

 the embryonic axis can be accurately observed and there is 

 normally not enough yolk digestion or absorption of albumen 

 to interfere with the identification of the long axis of the yolk. 



Pigeon No. j. This was an old bird at the outset to judge 

 from her subsequent activity. The first egg was laid March 16, 

 1910, the last January 14, 1912; ten weeks later she was killed 

 and a normal resting ovary found. During these 22 months 

 she laid 29 clutches without a single abnormal or infertile egg. 



TABLE I. (Pigeon No. 3.) 



