MATURATION OF OVUM IN KCHINUS ESCULENTU.S. 195 



frequently, when the section is through the side of it, the 

 centrosome is not cut at either end. There is no central 

 spindle. The chromosomes extend through the whole 

 equatorial plate, and the peripheral rays of the asters are 

 seen interdigitatiug opposite the equator. The fibres of the 

 spindle itself are somewhat uneven, and in relation to the 

 chromosomes there are darker bundles apparently of several 

 fibres spun together. As metakinesis proceeds the waviness 

 of the fibres becomes more distinct. The central centrosome 

 becomes flattened, but I cannot determine the manner in 

 which a division takes place. The outer centrosome 

 diminishes in size. Its central astral rays shorten, fig. 19, 

 and ultimately disappear as the apex of the spindle is 

 protruded. The lateral rays are obliterated progressively 

 until the point of the spindle stands clear. The first appear- 

 ance of the protrusion of the polar body is a tiny elevation 

 into which the end of the spindle is directed. Later, when 

 the spindle has risen to the height of the equatorial plane, 

 there is seen a depression on the surface of the egg where the 

 constriction takes place, and in which afterwards the polar 

 body lies. The rise of the spindle and its protrusion are very 

 difficult to explain. It remains approximately of the same 

 length throughout, and I do not see any special development 

 of the central aster over the polar one. Wilson (1900) finds 

 evidence in the protrusion of the spindle in favour of 

 Diiner's (1895) theory that the divergence of the poles of the 

 spindle in mitosis is due to the progressive elongation of the 

 central spindle. In Echinoderm ova, neither before nor after 

 fertilisation, is there a central spindle spun out between the 

 centrosomes, but it is probable, according to Wilson, that the 

 difference is only a secondary one, and that the spindle 

 consists in part of continuous fibres, and the waviness of the 

 spindle-fibres in the metakinesis would speak for the pushing 

 hypothesis. In any event, I cannot see how any hypothesis 

 founded on mechanical principles, such as illustrated in 

 Heidenhain's model, can explain the peculiar circumstances 

 of the polar mitosis. 



