BILATERALITY OF THE PIGEON S EGG 



301 



with the embryonic axis and then the angle was read at the center 

 of the glass slip. The probable error in measuring was found to 

 be from two to three degrees and in the measurements the aver- 

 age of three determinations was taken. 



The series of measurements so made showed that there is much 

 greater variation than has been supposed in the pigeon's egg, 

 but that it is not so great as Rabaud ('08) found in the hen's egg 

 (see footnote p. 276). This variation is not due to any change 

 of angle as the embryo develops, for in the eggs of a given bird 



Diagram III A polar view of an incubated egg illustrating the extremes of varia- 

 tion in axis angles in the pigeon. Complete inversion (180 degrees) involves 

 other factors and is not a simple variation, so it is not considered here. 



the average of angles measured after thirty-six to forty-eight 

 hours of incubation was the same as after eighteen to thirty-six 

 hours. The same conclusion was reached by Rabaud for the 

 hen's egg after a series of direct observations. Most of the 

 observations recorded here were made on embryos from three to 

 twelve somites. All workers have mentioned a certain amount 

 of variation, even to 180 degrees. In the pigeon, only four such 

 cases of complete inversion have come to my attention; one 

 was observed by Dr. Patterson one by Dr. Blount and two by 

 myself; that is, four out of more than six hundred careful obser- 

 vations showed that that end of the long axis which is usually 

 the infundibular one, went down the oviduct first. Aside from 

 these, the extremes of variation observed were 8 degrees and 



