CELL-ORGANS OF SEA-URCHIN EGG 585 



bipolar radiations are here, as elsewhere, signs of movement and 

 currents in the cytoplasm, which are stronger, at this time, at 

 the poles of the nucleus, where the relative surface of the absorb- 

 ing membrane is greater. As above stated, the segmentation 

 nucleus becomes oval. At the time of the development of the 

 bipolar radiations the basophilic chromatic substance is usually 

 in greater part absorbed by the nucleus. Therefore, the bipolar 

 radiations are not due to the movements of the cytoplasmic 

 chromatic substance. Their appearance seems to be connected 

 with the shifting of a substance corresponding to the plastosomes 

 of Meves and Duesberg, or to the archoplasm of McClung and 

 other authors. Accumulations of archoplasm around the poles 

 of the nuclei contribute to the formation of the structures known 

 as centrosomes and centrioles. These cell organs, if they may 

 be so called have a definite time limitation for the beginning of 

 their existence. No relation between the centrosomes of the 

 segmentation nuclei of the sea urchin eggs and the centrosomes 

 of the spermatozoon can be traced. Lillie's interesting experi- 

 ments seem to strengthen this position. Lillie recently centri- 

 fuged fertilized eggs, whereby merely small parts of the sperma- 

 tozoon heads remained in the egg, a considerable part of the head, 

 the middle piece, and the tail having been withdrawn. These 

 eggs continued to develop and centrosomes differentiated. The 

 centrosomes of the first segmentation nucleus do not appear in 

 well fixed preparations as independent bodies. They appear, on 

 the contrary, as dense cytoplasmic nets, in which granules some- 

 times accumulate (fig. 26). The centrosome is usually small, 

 while the spindle develops. 



When the spindle is fully developed, and the chromosomes, 

 form the equatorial plate the cytoplasm itself does not contain 

 any such differentiated structures as plastosomes or chromatic 

 granules. The stage of the equatorial plate is the expression of 

 physico-chemical reactions fulfilled between the achromatic sub- 

 stance of the nucleus and the absorbed chromatic cytoplasmic 

 substance and further between the differentiated chromosomes 

 and the archoplasm growing into the nucleus. How intimately 

 connected at this time are the spindle and the chromosomes, is 



