34 THE MATURATION OF THE EGG OF THE MOUSE. 



6. Abstriction of First Polar Cell. 



The process of abstriction begins as soon as the daughter chromo- 

 somes have come close to the poles of the spindle and the " Zwischen- 

 korperchen" have attained the condition shown in fig. 17. While the 

 spindle may sometimes be perpendicular to the surface of the egg, as 

 already stated, one pole lying in an elevation or protrusion, the condi- 

 tions indicate that, in most cases at 

 least, the spindle is either parallel or 

 oblique to the surface (figs. 15, 16,17). 

 The pole nearer the surface does not 

 at first lie in the middle of the protru- 

 sion, but at one edge of it (fig. 17). 

 The constricting process begins on 

 the side nearest the " Zwischenkor- 

 perchen," where in the surface of the 



(Compare figs^toa to i6d, plate 3.) e ^ a dee P' shar P S roove brin S S the 



vitelline membrane into contact with 



the " Zwischenkorperchen " of the side of the spindle nearest the surface. 

 The same condition exists also in fig. 16, in which the plane of sectioning 

 is very oblique to the axis of the spindle, as may be seen by comparison 

 with fig. H, which is a diagrammatic reconstruction of an imaginary 

 section of the egg in a plane perpendicular to that of the actual sections, 

 but parallel to the axis of the spindle. (Compare plate 3, figs. i6u 

 to i6d.) 



No other stage between this and that shown in fig. 18 having been 

 found, the further steps in the process can only be inferred. However, 

 it is highly probable that the contact between the vitelline membrane 

 and the "Zwischenkorperchen," shown in fig. 17, advances until it has 

 quite encircled the spindle. The result is that the entire periphery of 

 a disk-like body formed by the fusion of the "Zwischenkorperchen" 

 is finally in contact with the vitelline membrane (fig. 18), and the orig- 

 inal protrusion, now become more voluminous and containing the super- 

 ficial group of chromosomes, is thus separated from the egg. The inter- 

 zonal filaments, brought into a more nearly radial position during the 

 constriction, form the bulk of the neck of the polar cell. A little later 

 the constriction is completed by the ingrowth of the cell membranes of 

 both egg and polar cell in such a way as to cut off the interzonal fila- 

 ments and leave the "Zwischenkorperchen" on the outside of the cell 

 membranes of both polar cell and egg. Thus is formed the first polar 

 cell and the oocvte of the second order. 



