No. I.] THE EGG OF UMAX AGRESTIS. 221 



and 37). This process takes place before the breaking down 

 of the nuclear membranes. The intimate connection between 

 the chromatin and the nuclear filaments before the loss of the 

 nuclear membrane seems to support the view held by Flemming, 

 Reinke, and Wilson, that the chromatin is in contact with the 

 mantle fibers from the beginning, and not secondarily brought 

 into connection with them by the ingrowth into the nucleus of 

 the polar rays. If, however, the spindle is formed entirely by 

 a morphological rearrangement of the nuclear filaments that 

 are focussed at the centrosome, it is not wholly clear why the 

 nuclear membrane on the side toward the aster should be flat- 

 tened as if by pressure exerted on the membrane by the astral 

 rays. Moreover, the explanation of the formation of the spin- 

 dle by a rearrangement of nuclear substance does not account 

 for the formation of the second maturation spindle of Limax 

 agrestis. The second maturation spindle is formed within the 

 centrosphere (PI. XI, Fig. i6), while the old astral rays still 

 persist. The new asters are formed by rays that reach through 

 the centrosphere out into the cytoplasm, so that immediately 

 surrounding the centrosphere two sets of radiating fibers can 

 be detected — those that belong to the central astrosphaere of 

 the first maturation spindle, and those that belong to the asters 

 of the second maturation spindle in process of formation. 



It is not possible to explain the formation of the second 

 maturation spindle on any theory of a mere rearrangement of 

 a preexisting structure in Wilson's sense. The process seems 

 to be exactly the reverse of focussing ; for the rays are pro- 

 jected from the centrosome out into the centrosphere, which 

 appears almost structureless. Meanwhile the centrosphere in 

 which the spindle lies is constantly expanding and acquiring an 

 ever increasing diameter until it finally fades out. The astral 

 rays seem to be formed de novo about the centrosome ; hence 

 Wilson's explanation of the organic growth of the astral rays 

 may apply not only to their continued growth, but also to their 

 origin. While the spindle is forming, the chromatin still lies 

 outside the sphaere, on the upper surface, where it was left 

 after the extrusion of the first polar body (PI. XI, Fig. 15). 

 Later the chromatin is drawn into the equator of the spindle. 



