BILATERALITY OF THE PIGEON S EGG 



275 



III. THE PROBLEM 



Attention has already been called to the relation between the 

 chalazal and embryonic axes in the eggs of the hen and pigeon 

 and the matter is considered in detail in section VI. The general 

 character of the relation in the pigeon is shown in diagram I. 



Diagram I A polar view of an incubated pigeon egg ('yolk') showing the rela- 

 tions between the embryonic and long (chalazal) axes. The end of the egg which 

 was directed toward the blunt end of the shell, where the air chamber is always 

 found, is toward the left. The end which passed down the oviduct first, with the 

 'cloacal' chalaza (c.c.) attached, is to the right and the head of the embryo is 

 away from the observer; c.c, cloacal chalaza. 



The essential feature is that when the end of the egg opposite 

 the air chamber of the shell is held to the right, the head of the 

 embryo is away from the observer. The chalaza at this end is 

 frequently heavier, double, and more firmly attached than the 

 other; it is in fact the chalaza which is formed first, since the end 

 of the egg to which it is attached goes down the oviduct first. 

 This chalaza may accordingly be called the 'cloacal chalaza' 

 (c.c.) for its end of the egg, in the oviduct and in the ovary as well, 

 is nearest the cloaca; the other will be referred to as the 'infundib- 

 ular chalaza.' It is evident that one end of the chalazal axis is 

 different from the other with reference to the embryo, that is, 

 the chalazal axis is definitely related to the embryonic axis of 

 bilateral symmetry. Furthermore the chalazal axis is the longest 

 axis of the egg. This fact has not been recognized hitherto 



