SO-CALLED PARTHENOGENESIS IN THE WHITE MOUSE. 245 



that neither centrosomes nor polar radiations are present. In 

 my own preparations radiating fibers are to be seen at the poles 

 of a number of second polar spindles. As in the case of the 

 first polar spindles, however, these are not to be considered true 

 asters, but spindle fibers which have broken away from the 

 spindle, and have assumed a radial position at the ends of the 

 spindles. This will be discussed more in detail later. In general, 

 centrosomes are absent in second polar spindles of atretic oocytes, 

 but are present in some cases. When they are seen, they con- 

 sist of a few minute granules which stain deeply and are either in 

 a compact group at the poles of the spindle or are somewhat 

 spread out forming a sort of cap for the spindle. (See Figs. 3, 

 4 and 6.) 



The first polar body is almost always present with the secon- 

 dary oocyte in atretic follicles, although in a few instances the 

 spindle has all the characteristics of a second polar spindle while 

 the polar body is not to be seen. In such cases it is possible 

 that the polar body has already degenerated and been absorbed, 

 or as Kirkham suggests for normal oocytes, it may have been 

 "forced through the zona (pellucida) by the contraction of the 

 latter under the influence of changing osmotic conditions" 

 during fixation. In nearly every instance, however, the polar 

 body is present, lying within the zona pellucida, and somewhat 

 flattened between the oocyte and the zona. The smaller dimen- 

 sion of the polar body is one half or two thirds of the larger, 

 while the larger diameter itself is a fifth to a third that of the 

 oocyte. In a few cases the polar body contains a spindle more 

 or less deranged and abnormal (see Fig. 3) but usually the 

 chromosomes are scattered through it irregularly. They may 

 be grouped in a few large irregular masses of chromatin or there 

 may be a number of smaller chromosomes of abnormal size and 

 shape. In a few cases a resting nucleus may be seen in the 

 polar body (Fig. 10). The second polar spindle is usually 

 found in the oocyte near the polar body, at right angles to the 

 radius of the egg-cell. Rarely it may be seen in the other side 

 of the oocyte, and a few spindles have been seen in a radial 

 position. A few instances were observed where the oocyte con- 

 tained two spindles; this is probably due to the fact that the 

 egg-cell had two nuclei to start with. 



