The Early Development of the Pigeon's Egg. 3 



My thanks are dne to Professor Whitman and other members of 

 the Department of Zoology for a fellowship and an assistantship, 

 which has made it possible for me to carry on this research. Pro- 

 fessor F. R. Lillie, who suggested the problem, has followed the 

 work with helpful interest, and I am particularly indebted to him. 

 Professor W. L. Tower has given indispensable help in the technique 

 of photography, and Professor C. M. Child has helped me with some 

 literature. Figs. 5, 6, 9, 11 and 14 are the work of Mr. Kenji Toda. 



I. Methods. 

 Following the methods of workers who have preceded me, the 

 blastoderm has been killed and hardened on the yolk and the orienta- 

 tion marked with a bristle. Immediatelv after a window has been 



b 

 Fig. 1. — Diagram to show the method of marking the orientation. The 



arrow indicates the direction of the axis of the future embryo, b, bristle. 



made through the shell, a bristle is inserted in the side of the yolk 

 toward the blunt pole of the shell. Later (usually when the egg 

 is in 70 per cent alcohol) a iive-sided piece, including the blastoderm, 

 is cut out from the yolk. One side of the five-sided area is per- 

 pendicular to the chalazal axis and is toward the large pole of the 

 egg. Two sides are parallel to each other and to the chalazal axis, 

 and the last two sides meet in a sharp angle pointed toward the small 

 pole of the egg. Fig. 1 explains this orientation, the anterior side 

 of the blastoderm being toward the point of the arrow. This five- 

 sided block is easily seen in the paraffin cake for orientation in 

 cutting. In some eggs there is little difference between the blunt 

 and sharp poles of the shell, and in these the orientation is uncertain 

 after the egg has been taken out of the oviduct. While the egg 



