PRINCIPAL AXIS OF SYMMETRY IN THE BIRD S EGG. 



321 



have few data as to the relation of the two axes in birds other 

 than the hen and pigeon but such observations as we do have 

 indicate that many if not all birds' eggs show the same general 

 relations. Duval in 1884 assumed this to be true but he never 

 published any evidence. 



The principal egg axis is defined by the form of the secondary 

 egg envelopes. In addition to the obvious difference between the 

 two ends of the shell, the ends of this axis are differentiated by 

 even more constant characters like the position of the air space, 

 pigment distribution in certain forms, and less dependable 



lig.alb. 



FIG. i. A diagrammatic polar view of an entire pigeon's egg showing the rela- 

 tion between the embryonic axis and the various features of the principal egg axis. 

 The right side of the embryo is nearer the end of the egg which passed first down the 

 oviduct, i. e., the pointed end of the shell. In the "uterus" the pointed end of the 

 shell is directed toward the cloaca, the blunt end with the air space toward the 

 infundubulum of the oviduct. The principal egg axis, ( ) is marked by the 

 major axis of the shell, the air space (a.sp.), the ligamentum albuninis (lig.alb.), 

 the chalazal axis (c.c., cloacal chalaza), and the long axis of the ovum. 



characters such as the size differences between the chalazae. 

 All of them are expressions of the structure and activity of the 

 oviduct and their relations are constant because the egg is 

 definitely oriented in the oviduct. As this point of view is new 



