PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 323 



usually at both ends of the shell but this prolongation through 

 the outer thin albumen is more marked at the pointed end in 

 the laid egg. This prolongation Tredern fancifully called a 

 "ligament." 



7. Long Axis of Yolk. The striking fact that the yolk (ovum) 

 is not a sphere has been generally overlooked. One of its axes 

 is greater than either of the other two and this axis is perpendicu- 

 lar to the polar axis and coincides with the principal axis. 



8. The Oviducal Orientation. All of our evidence points un- 

 mistakably to the conclusion that the pointed end of the egg 

 passes first down the oviduct. We may therefore speak of a 

 cloacal and an infundibular end of the egg. The principal axis 

 therefore, coincides with the oviducal axis and the differences 

 which we find between the two ends of the former are due to 

 differences in the activity of the oviduct. The long axis can be 

 identified in all ovarian oocytes and at the time of ovulation 

 the ovum is oriented in the oviduct with reference to its long 

 axis. The long axis accordingly determines the principal axis 

 as we see it in the laid egg. These relations are of fundamental 

 importance for they demonstrate that the embryo is definitely 

 related to an ovarian axis of symmetry. 



9. I was able to show that the end of the long axis which is to 

 pass first down the oviduct is predetermined in the ovary by 

 the position of the latebra of Purkinje. 



10. Finally the eccentric position of the latebra is determined 

 by the corresponding position of the germinal vesicle in young 

 oocytes. 



In view of these facts we may express the relation between 

 egg and embryo thus : The right side of the embryo is nearer that 

 end of the principal egg axis which is predetermined in the ovary 

 to pass first down the oviduct. This holds of course only for stages 

 previous to the turning of the embryo on its side. To state the 

 case in the more usual way we should say: If the end of egg which 

 is predetermined to pass first down the oviduct be held to the 

 observer's right the head of the embryo will be directed away 

 from him. The presence of an axis of symmetry in the ovarian 

 oocyte so definitely related to the embryonic axis naturally led 

 me to look for morphological evidences of the embryonic axis 



