124 George Lefevre 



2 Origin of the Cleavage Nucleus and Amphiaster 



After extrusion of the second polar body, the egg nucleus moves 

 toward the center of the egg and soon loses its irregular con- 

 tour by the complete fusion of the chromosomal vesicles v^hich 

 entered into its composition. There now follows a long resting 

 period, and during this pause, which may last for from one to two 

 hours, or even longer, the nucleus increases considerably in size 



The first indication of the preparation for cleavage is seen in 

 the simultaneous appearance of two delicate asters which are 

 situated opposite each other and immediately outside the nuclear 

 membrane (Fig. 27). They lie in a plane approximately perpen- 

 dicular to the axis of the egg, and in the center of each aster a dis- 

 tinct centrosome is visible. There is not the slightest evidence 

 that the cleavage asters, for such these are destined to be, are 

 derived by division of a single primary one; when they are first 

 seen they lie at a considerable distance from each other, usually 

 on opposite sides of the nucleus. There is, furthermore, no doub- 

 ling of the centrosomes at this time, and no intermediate stages are 

 found which would indicate a division and separation of the asters. 

 Fig. 27 illustrates the almost invariable condition of the egg 

 nucleus when the asters are first discovered. My observations 

 agree with those of Wilson ('01) on the magnesium eggs of Tox- 

 opneustes in that they show clearly that the cleavage centrosomes 

 lie outside the nuclear membrane but immediately upon it, 

 although Wilson is strongly inclined to believe, from the evidence 

 presented by his sections, "that the cleavage amphiaster arises by 

 division of the single center" (p. 564). He first observed a single 

 centrosome, surrounded by a conspicuous aster, lying outside the 

 intact nuclear membrane at one pole of the nucleus; all inter- 

 mediate conditions were found between this stage, through a 

 monaster stage, to the complete establishment ot the dicentric figure, 

 and, while recognizing the possibility of being misled by the nature 

 of the material, he felt justified in regarding the amphiaster as the 

 product of division of the primary monaster. As the evidence on 

 which he bases his conclusion for Toxopneustes is not presented 



