PRINCIPAL AXIS OF SYMMETRY IN THE BIRD S EGG. 



343 



TABLE II. (Pigeon No. 4.) 



This record doubtless includes the bird's entire period of ac- 

 tivity. The falling off during the second year was due to the 

 period between matings (the active months of February and 

 March) and the hatching of four clutches later in the spring. 

 As it was she laid 51 clutches in the course of four years; 12 per 

 cent, of the eggs were abnormal but most of these latter were 

 laid during the last year. 



The curve of variability (Fig. 3, 4), has a single mode when 

 10 classes are used but with five degree classes there is one mode 

 at 70 another at 85. There is a total variation of 53 from 

 52 to 105. 72 per cent, of the eggs measured fall between 65 

 and 85. Of the twelve eggs which diverged most widely from 

 the mean 9 showed irregularities in the chalazae, the others were 

 quite typical. One egg which showed an angle of 35, twenty 

 degrees lower than any other egg of this bird, was not included 

 in the data plotted, as the long axis was obliterated by yolk 

 digestion. Another egg which showed no well defined long axis 

 had an angle near the other extreme of variation, namely 95. 

 The long axis was distinct in all the other eggs of no. 4, as may 

 be seen from two examples: 



Egg 399 (n somites) 21 mm. (long axis), 19.0 mm. (transverse axis), 19.0 mm. 



(polar axis). 

 Egg 479 (primitive streak) 22.0 mm., 19.4 mm. (transverse axis), 18.0 mm. (polar 



axis). 



Two curves plotted from the eggs laid during the first and 

 second matings are almost identical and so this case affords no 

 evidence that the male has any effect upon the relation of embryo 

 to principal egg axis. 



The records of the remaining three pairs are more complete 

 as they were the offspring of the above pairs. Their relationships 

 are as follows: 



