32 THE MATURATION OF THE EGG OF THE MOUSE. 



usually smooth in appearance and of uniform size from end to end (figs. 

 8a, 86, 9, 10, n). A little later they often exhibit minute, granular 

 thickenings at irregular intervals along their lengths (figs, n, i$a, 136, 

 140). The polar ends of the fibers become thickened and more or less 

 confluent in the later stages (IV6 and V; figs. 12, 130,, 136, 14, 15), 

 frequently to such an extent that the end of the spindle looks homo- 

 geneous, and the fibers are distinguishable only as faint striations (fig. 

 15). In some cases the attachment of some of the fibers to chromosomes 

 is evident (figs. 10, n, i$a, 136, 14, 140). In addition to these fibers 

 there are, however, others, very delicate ones, running from pole to pole 

 without being attached to any chromosome (figs. i$a, 136). These 

 probably constitute a part of the interzonal filaments. The latter, when 

 the daughter chromosomes have separated, are very fine (fig. 15). Later 

 (figs. 1 6 and 17) they become thicker, and in the telophase (fig. 18) they 

 apparently become fused into a pale, nearly homogeneous, faintly stri- 

 ated bundle, lying between the two deeply stained masses resulting 

 from the confluence of the chromosomes. The chromosomes, drawn 

 nearly to the end of the spindle, lie in a somewhat deeply staining matrix 

 (fig. 17), which is perhaps derived from the spindle fibers. 



At the middle of each interzonal filament is a thickening, a "Zwi- 

 schenkorperchen. ' ' The number of these was not determined. The thick- 

 enings, at first elongated, become more globular (fig. 17), and at length by 

 fusion give rise to the "cell plate" (fig. 18), a disk-shaped mass staining 

 moderately deeply. The further fate of the interzonal filaments and the 

 "Zwischenkorperchen" will be discussed later (pp. 34 and 43). 



4. CENTROSOMES, CIRCUMPOLAR BODIES, AND CLEAR REGION. 



Although recently the existence of centrosomes in connection with 

 the first maturation spindle in the ovum of the mouse has been asserted, 

 the evidence, so far as our preparations show, points clearly to the entire 

 absence of centrosomes. Not even in the two cases illustrated in plate 2, 

 figs. 10, n, is there any hint of a centrosome at the ends where the fibers 

 converge to a point, although there are clearly a few fiber-like radiations 

 in the surrounding cytoplasm. If there were any centrosomes present, 

 one would expect them to stain as sharply as those in the surrounding 

 follicle cells during division. In the eggs from which figs. 10 and n were 

 drawn there are no polar radiations except those figured and mentioned 

 above, nor have any other instances been observed in which there were 

 polar radiations as marked as these. Occasionally a few fibers may be 

 observed outside the limits of the spindle (figs. 9, 11, 12, 136) and ex- 

 tending from the poles obliquely toward the plane of the equator. 



The two conditions mentioned as characteristic of Stage IV6 are 

 the circumpolar bodies and the clear region around the whole spindle. 

 The two arise at about the same time and likewise disappear together; 

 they both reach their greatest prominence at the stage when the chro- 

 mosomes divide at metakinesis. 



