16 ART. 10. — N. YATSU : 



had been removed at the 2-cell stasje (pp. 5, 6\ The above 

 examples send to show, if T mistake not, that the germinal 

 localization and cleavage factors are not so closely related as we 

 have thought. 



As regards the relation between promorphology and cleavage 

 pattern it cannot be doubted, as almost all investigators agree, 

 that there is a direct connection. It may here be added that 

 what I have stated under sections VIIT and IX seems to 

 •indicate the importance of the polarity in relation to cleavage 

 pattern : the cleavage goes on normally in all blastomeres^ (4, 6 

 or 8 in number) that have arisen by vertical cleavage. 



Now as to the cleavage factors. The regular cleavage pattern 

 is the result of a regular succession of internal stimuli, which deter- 

 mine the position of the spindle and consequently the relative size of 

 the blastomeres. The factor of directing cleavage planes mani- 

 fests itself quite early, for instance, by the position of the cleavage 

 centres and in Ascaris still earlier by the " horns " or portions 

 of chromosomes projected from the nucleus, as noticed by zur 

 Strassen ('06, p. lo4). The factor controlling the position of the 

 spindle, on the contrary, seems to come into effect much later 

 than that of direction. The spindle may move about in the 

 blastomeres like a boat carried to and fro by the waves, when the 

 normal relation is disturbed by the removal of a certain portion of 

 the cytoplasm. As we have already seen (see pp. 2, 6, 12), we have 

 good reason to believe that there is no predetermined protoplasmic 

 differentiation for fnture diastems in the egg. zitr Strassen ('00, 

 p. 145) suggests, with some reserve, that in the egg of Ascaris 

 the position of the spindle may be determined by preexisting 



1 One may argue ih^t this is simii]y due to similar distriljution of materials in each 

 blastomere. That may be so in both sea-urchin and Cerebratulus but certainly not in Crepidida. 



