274 



N. YATSU. 



furrows have began through the middle, while above one spindle lying 

 obliquely to the surface the cleavage first proceeded perpendicularly to 

 the surface and then changed its course to meet the spindle at the right 

 angles. 



The above two cases seem to indicate that the cleavage goes on per- 

 pendicular to the middle point of the spindle irrespective of the position 

 of the asters 1 . 



5. One-sided Constriction. 



Cases L, M. In the compressed eggs the first cleavage is often ac- 

 complished by an one-sided constriction (Figs. 24, 25) as in the egg of the 

 coelenterates and of Pctromyzon. This mode of division is met with very 

 seldom even among those under normal conditions. As has been main- 

 tained by Ziegler the one-sided cleavage is certainly due to the excentric 

 position of the segmentation nucleus. In compressed eggs the nucleus 

 seems to find it more difficult to attain the central position. In artificial 

 parthenogenesis this mode of cleavage is very common, since the egg 

 nucleus usually lies near the animal pole (cf, Morgan '99 p. 452 Fig. 2). 



It is, therefore, certain that the occurrence of the one-sided constric- 

 tion is not limited to the coelentrates and Pelromyzon but abnormally it 

 may take place in other forms, where, under normal conditions, the con- 

 striction is accomplished by both the animal and vegetal furrows. 



6. Formation of the Daughter Nucleus independent 

 of the Centrosome. 



Cise N. Among compressed eggs I found one, in which one of the 

 asters has separated from the chromosomes at the anaphase (Fig. 26). 

 Should the separation take place a little earlier, say, at the metaphase, all 

 the chromosomes would have been attached to one aster, which case, in fact, 

 was met with very often. In the present case, however, the aster had sepa- 



i STRASBURGKR h is obs.rved that the cleavage furrow cut in obliquely to the spintile ('80 p. 

 261). CONKLIN found that sometimes in the egg of Crepidula cleavage gajs on without res- 

 pect to ths position of the spindle ('03 p. 94). This is the only exception so far as I know, and I 

 think this may be due to the w II fixed localization peculiar to this form. 



