PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 355 



considerably, although extreme irregularities are rare (3 per 

 cent.). Since the two chalazse were indistinguishable in only 28 

 per cent, of cases we may say that in the majority of eggs more 

 albumen is secreted below the ovum than above it when it 

 first enters the duct and consequently the cloacal (pointed end) 

 chalaza is frequently larger than the other. Unfortunately I 

 have recorded only 14 of the instances when this was found to be 

 the case in early oviducal eggs. 



V. GENERAL CONCLUSIONS. 



1. The older literature on the bird's egg has been grossly 

 neglected by recent students of this subject. 



2. The bilaterality of this egg finds expression in two axes of 

 bilateral symmetry: the embryonic axis and the principal egg 

 axis which is defined for the most part by characters of the 

 secondary egg envelopes. 



3. These axes are definitely related to each other in such a 

 way that the right side of the embryo is nearer one of the ends 

 of the principal axis than the other. 



4. This holds for the eggs of all the species that have been 

 examined with this relation in mind: viz., the domestic fowl, 

 the common pigeon, the emu, the duck and perhaps the sparrow. 



5. The actual angle between the two axes (axis angle) is subject 

 to great variation. 



6. Conclusions from observations on the pigeon's egg. 



(a) The axis angle was found to vary almost 180 but 85 per 

 cent, of the cases fell between 45 and 90. 



(6) Eggs obtained from a single bird vary much less, the 

 maximum variation being from 32 to 68 per cent, of that found 

 in eggs from the entire flock. 



(c) Extreme variations of the axis angle are probably due to 

 imperfect orientation of the ovum in the oviduct, to abnormali- 

 ties in its action or to irregularities during the final growth period 

 of the oocyte. Such eggs usually show irregularities either of 

 long axis or chalazae. 



(d) The angle in normal eggs of a given bird, while usually 

 restricted to a range of 45 is apparently determined within this 

 range by the laws of chance during oogenesis. 



