2l8 BYRNES. [Vol. XVI. 



the chromatin is brought into the equatorial plate, the separate 

 origin of the maternal and paternal chromatin is no longer 

 recognizable. 



When the astrosphaeres of the segmentation spindle first 

 form, the centrospheres show no differentiation into zones such 

 as has been described for the centrospheres of the maturation 

 spindles. After the spindle has fully formed, however, the 

 "cortical" and "medullary" zones begin to develop around 

 the centrosome, so that the astrosphaeres that appear in con- 

 nection with the sperm-nucleus are generally structures similar 

 to those of the maturation spindle. 



When the segmentation spindle is completely formed and 

 the Qgg is ready to divide, the spindle lies transverse to the 

 polar axis in the upper hemisphere in a mass of protoplasm 

 that stains more deeply than the rest of the egg. After the 

 maturation spindle has divided, and the two resulting daughter- 

 nuclei have come to rest, all trace of the centrosomes and asters 

 is lost. When the daughter-cells are again ready to divide, the 

 centrosomes and asters reappear on the periphery of the nucleus, 

 and gradually separate to form the segmentation spindle of the 

 next division, and so on. 



III. Observations on Abnormal Ova. 



I . Polyspermy. 



I hoped to find in polyspermic eggs a clue to the origin of 

 the sperm-asters. Polyspermy rarely occurs in Limax, how- 

 ever, and when cases of it are found they throw no light on 

 the origin of the centrosomes. I have found single capsules 

 containing as many as twenty-five eggs, some of which were 

 unsegmented and apparently normal ; others had divided into 

 two cells ; still others into four cells. The unsegmented eggs 

 when killed in Vom Rath's fluid and mounted whole, unstained, 

 showed as many as three small vesicular sperm-nuclei in addi- 

 tion to the usual large egg-nucleus. One set of abnormal eggs 

 suspected of being polyspermic, when sectioned, showed that 

 in some cases as many as twelve spermatozoa had entered a 



