PRINCIPAL AXIS OF SYMMETRY IN THE BIRD S EGG. 351 



exceptionally variable. The other three birds laying an aggre- 

 gate of 204 eggs had no inversions. This gives a clue to the 

 reason for the great difference in the number of cases reported for 

 the hen. We are again dealing with a matter of individual 

 variation. It is manifest that we should exclude from our studies 

 involving axis angles all birds which show a tendency to vary 

 over 25 from a mean. 



How are we to interpret the phenomenon when it does occur? 

 Is it due to an inversion of relations in the ovary or is it more or 

 less accidental? It is always possible that a normal ovum may 

 be unable to orient itself with reference to gravity before oyula- 

 tion owing to pressure from neighboring ova, adhesions of the 



FIG. 4. A diagrammatic polar view of a pigeon's egg showing an "inverted" 

 embryo. This type of inversion can be explained (egg 479' of no. 4). In this 

 particular case the inversion was very probably due to the fact that the end of the 

 long axis of the yolk which should have entered the oviduct first, entered last. 



oviduct, etc., and so the end of the long axis which should have 

 entered first enters last. It may also have fallen into the ccelom 

 at first and have subsequently entered the duct inverted. Since 

 we have complete records of no. 4 and no. 5, the only birds I 

 ever had which laid inverted eggs there is a basis for something 

 more than speculation. When I opened egg 479' of no. 4 and 



