272 



N. YATSU. 



and can no longer be altered by injury of the polar region of the egg. 

 This result agrees well with cytological 'data, which will be mentioned 

 elsewhere in detail. 



3. Division after the Injury of the Pole-rays. 



In the cases C. G. H and I (Figs. 7, 15-20) we have seen that irres- 

 pective of the injury of the pole-rays due to the operation the cell-division 

 took place normally, though the resulting blastomeres were of unequal 

 size, excepting the case C (Fig. 3). If we assume that the pole rays al- 

 one play the most important part in cell-division as maintained by the 

 advocates of the muscle-fibre theory, it is well nigh impossible to account 

 for the above result. The establishment of the diasteme seems to be no 

 less important in this respect. 



Case A (Fig. 1) affords an intesting fact in this connection. In this 

 egg the centres of one pole have been cut apart from the chromosomes at 

 an anaphase. I Despite this the cleavage took place normally. 



This clearly shows that a constriction can be accomplished, if not 

 initiated, by a centrosome at one pole. This seems to give a very strong 

 support to Boverl's statement that the cell-division may be performed by 

 one ray-system only ('04 p. 5). 



4- Relation between the Spindle axis and the Cleavage plane- 



In order to subject eggs to a constant pressure they were put in a com- 

 pressorium since under a cover glass the pressure increases gradually as 

 the water evaporates. Several egg thus treated yielded interesting 

 anomalies. Of these two will be mentioned in this section. 



Case J. The egg is at the four-cell stage (Fig. 21); the third cleavage 

 mitoses are at an anaphases in three blastomeres, while in one this has 

 proceeded to the telophases. In the upper right blastomeres the spindle 

 is almost parallel to the surface and the cleavage is about to cut in to the 



I It is quite probable that On the operated side a partion of the mother centrosome has been 

 left in the nucleated fragment, but it can not be doubted that two new centrosomes were in the 

 enucleated one. 



