THE ORGANIZATION OF THE EGG OF UNIO, 



BASED ON A STUDY OF ITS MATURATION, 



FERTILIZATION, AND CLEAVAGE. 



FRANK R. LILLIE. 



TABLE OF CONTENTS. page 



Introduction ^^^ 



I. The Behavior of the Spermatozoon in the Egg up to the 



Time of Extrusion of the Second Polar Globule 229 



1. The Sperm-Nucleus 229 



2. The Sperm-Aster and Amphiaster : Origin and Disappearance.... 231 



a. Observations ^3^ 



i>. Literature ^33 



IL The Maturation of the Egg 235 



1. The Chromosomes ^35 



2. Achromatic Structures 23" 



III. Growth, Migr.\tion, and Union OF the Germ-Nuclei; Behavior 



of the Sphere Substance; Origin of the Cleavage Cen- 

 trosomes ^45 



1. Observations ^45 



2. Literature and Theory of Fertilization 250 



IV. Movements of the First Cle.wage Spindle. First and Second 



Cleavages "55 



V. Concerning the Sphere Substance 256 



VI. Two Recent Theories of Unequal Cleavage 258 



VII. Organization of the Egg 261 



Introduction. 



This piece of work was originally undertaken in 1 896 as a 

 study of the behavior of the centrosomes in the fertilization of 

 the egg of Unto, but I was soon led into a general study of 

 the centrosomes in this egg. The results which I obtained 

 seemed to be irreconcilable with the views then current of the 

 functions of the sperm-centrosomes in fertilization, and with 



227 



