232 



MARY BLOUNT. 



presenting the abundant proof which the material affords for my 

 conclusions. 



Through the kindness of Prof. C. O. Whitman and other 

 members of the Department of Zoology, I have been the recip- 

 ient of a university fellowship which has enabled me to pursue 

 this study. Dr. F. R. Lillie, at whose suggestion I undertook 

 the research, has followed the work carefully, and I thank him 

 for his interest and kindness. I am also indebted to Mr. W. L. 

 Tower for his help in photography. The living egg is a difficult 

 subject, and it was only after a great many efforts that I secured 

 any photographs. Twelve cleavage stages have been photo- 

 graphed, although only three are presented in this paper. 



Methods. 



Following the method of workers who have preceded me, the 

 blastoderm has been killed and hardened on the yolk, and the 

 orientation marked with a bristle : Immediately after a window 

 has been 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 five-sided piece, in- 

 cluding the blastoderm, is cut out from the yolk. One side of 

 the five-sided area is perpendicular 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 



Fig. 1. Diagram to show the method of marking the orientation. The arrow in- 

 dicates the direction of the axis of the future embryo, b, bristle. 



sharp angle pointed toward the small pole of the egg. Fig. 1 \ 

 makes this orientation clear, the anterior side of the blastoderm 

 being toward the point of the arrow. This five-sided block is 



