FORMATION OF POLAR SPINDLE IN BUFO. 77 



at least twenty. Owing, probably, to their greater volume, these 

 larger masses always stain much more intensely than do the 

 small granules. Meanwhile the spindle has lost its uniform 

 diameter and has become much thicker in the middle where the 

 meshwork of fibers appears more distinct and more regular. The 

 spindle soon becomes barrel-shaped and its fibers are quite clearly 

 defined in the middle region but not at the poles (Fig. 2). The 

 radiation from the spindle disappears entirely except at the poles 

 where it forms distinct asters ; some of the rays are very long 

 and cross each other at the equator of the spindle. During its 

 migration towards the upper pole of the egg the spindle shortens 

 somewhat and gradually becomes more slender and pointed, a 

 phenomenon seen by Van Name (17) in the eggs of Planarians, 

 by Korschelt (12) in Opliryotroclia, by Griffin (8) in Thalassenia, 

 and by Boveri (i) in Ascaris. 



At no stage in the formation of the spindle or in its later history 

 can any centrosome be found in the polar asters. As the spindle 

 becomes more pointed, the rays converge more sharply at the 

 poles, but even when the radial systems are best developed (Figs. 

 2, 3), the rays appear to run into each other in the center of each 

 aster and there is not the slightest trace of any kind of a central 

 body. Carnoy and Lebrun (2) in their study of the batrachian 

 egg, Eismond (5) in his work on Siredon and Triton, Fick (6) in 

 studying the maturation of the Axolotl egg, and Sobotta (15, 16) 

 in working on the egg of the mouse and of Amphioxus, have all 

 failed to find a centrosome in the asters of the polar spindles. If 

 such a structure is normal in these eggs and also in the egg of 

 Bnfo lentiginosus, methods of fixation and staining which have so 

 clearly demonstrated its presence in other eggs are totally inad- 

 equate in these cases to show the slightest trace of it. 



At the stage of Fig. 3, the small chromatin granules have 

 entirely disappeared. Whether they have all gone into the 

 large chromatin clumps or whether some have been absorbed 

 by the cytoplasm cannot be determined. At this time the num- 

 ber of large chromatin masses still varies slightly in different eggs; 

 in some cases there are nine such clumps of chromatin, in others 

 at least fifteen. These chromatin masses are, for the most part, 

 scattered irregularly along the spindle fibers, occasionally, how- 

 ever, one or more of them can be seen entirely outside of the 



