PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 353 



periphery of the ovary, that the yolk might be so laid down 

 that the nucleus would approach the original vegetal instead of 

 the animal pole. Thus inversion would be accomplished. It 

 should be noted also that three of the inverted eggs of no. 5 

 were the first eggs of three successive clutches and a fourth was 

 laid soon after, all of them dating from the last six months 

 before the ostium was closed by adhesions. 



It is clear from these data that inversions are exceedingly 

 rare in pigeons' eggs, and that only certain individuals ever lay 

 such eggs. There are two types: those which are due to an 

 inversion of the long axis of the ovum at ovulation and those 

 which may be explained by assuming an inversion of the polarity 

 of the oocyte. It might be expected in such a case that one 

 should find situs inversus viserum but this was not noted in the 

 one instance in which the embryo was far enough developed to 

 reveal it (cf. Conklin, 1903). 



C. Variations in the Chalazce. The most accurate and con- 

 venient method of measuring the axis angle in an egg is to deter- 

 mine the relation between the chalazal and embryonic axes. 

 Patterson (1909) was, I believe, the first to use the chalazae for 

 this purpose and I found it by far the most satisfactory method. 

 It is important therefore, to see exactly how variable the chalazae 

 are and especially how frequently the chalazal axis deviates 

 from the long axis of the ovum which determined it as well as 

 other characters of the principal egg axis. A similar series of 

 observations should be made on the hen's egg where there seems 

 to be much more irregularity. 



Typically one finds the chalazae attached to the chalaziferous 

 albumen at the ends of the long axis of the ovum. As the animal 

 pole flattens out more and more during incubation the points 

 of attachment seem to approach the animal pole, still remaining 

 in the greatest circle of the elliptical ovum. It is rare to find this 

 condition in the pigeon during the first two days of incubation. 

 One often finds a cap or button of clear chalaziferous albumen at 

 the point where each chalaza is attached. Usually when the 

 infundibular (i. e., blunt end) chalaza lies free in the albumen or 

 has become invisible this button marks the corresponding end 

 of the chalazal axis. Again there are eggs which show both 



