PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 335 



from the other, to the youngest oocytes in the ovary. These 

 facts have been confirmed by my subsequent observations and 

 the complete demonstration for the pigeon requires only experi- 

 mental evidence . 



H. The Relation of the Embryo to the Principal Egg Axis (Axis 

 Angle}. Who was the first to describe this remarkable relation? 

 No one has ever tried to find out. Duval (1884) has often been 

 given credit for it, but he was only one of the few who have 

 published concrete data on the subject. These data and the 

 use he made of them are largely responsible for the deeply rooted 

 tradition of today that the embryo in the hen's egg lies at right 

 angles to the principal egg axis. It speaks volumes for the 

 mighty influence which the classic "Atlas d'Embryologie" has 

 had upon all of us. Duval himself gives the credit to Balfour 

 and Kolliker as Kionka also does in 1894, but neither of these 

 appears to have mentioned the matter except incidentally in 

 their text-books. Dareste (1891) and Rabaud (1908) attribute 

 it to v. Baer. v. Baer wrote one of the best and most complete 

 accounts that has ever appeared in the 1828 edition of his epoch- 

 making work. It is difficult to understand how this discussion 

 has come to be so completely forgotten. But v. Baer himself 

 speaks of the relation as a matter of common knowledge at the 

 time. The earliest mention I have succeeded in finding is in 

 Pander's thesis (1817). In the German edition illustrated by 

 D'Alton which immediately followed the original he says (p. 9): 



"Sobald die Langenpole des Embryo, welche nicht dem Ldngen- 

 sondern dem Querdurchmesser des Eyes entsprechen gesetzt sind, 

 entsteht zwischen den beiden Falten (des Primitifstreifens) der 

 Lange nach, ein zartes, oben rundlich, unten lanzettformig breiter 

 Streifen, das Rudiment des Ruckenmarkes." The casual nature 

 of the statement leads me to think that he was merely stating an 

 accepted fact which was not particularly significant for the matter 

 in hand. As many workers before the day of physiological salt 

 solution opened the egg at the broad end by way of precaution 

 it may not have been discovered much before this time. 



It is possible that Malpighi (1672) noticed the variability 

 between the principal egg axis and the embryonic axis. In all of 

 his figures the embryo is drawn with the head up on the page 

 and in most it is parallel to the sides of the page. In certain 



