CELL-ORGANS OF SEA-URCHIN EGG 583 



copulation. Chromatin particles appear in connection with the 

 nuclear membrane as numerous small, or a few large, accumula- 

 tions. The small accumulations may become free from the 

 nuclear membrane and may be found within the achromatic 

 substance of the nucleus as well defined bodies (fig, 19). Off- 

 shoots in form of thin threads are seen to grow from such chro- 

 matin bodies (fig. 19); they form a chromatin net, from which 

 the chromosomes are differentiated (fig. 28). 



Less frequently the fusion of the pronuclei takes place at a 

 time when the male pronucleus already possesses differentiated 

 chromosomes. No common net of chromatin is formed in this 

 case and the sperm and egg chromosomes remain entirely sepa- 

 rate during the whole time of their differentiation. 



As mentioned above, a bipolar radiation finally appears around 

 the two poles of the nucleus. The single radiation around the 

 nucleus seemed to be connected with the flowing of the baso- 

 philic substance toward the nucleus. The bipolar radiations 

 may sometimes appear at an early stage, when the segmentation 

 nucleus does not show differentiated chromatin. In other more 

 numerous cases chromosomes are in full development or even 

 already formed before the bipolar radiations appear. The bipolar 

 radiations seem to be connected with the development of the 

 spindle, since the threads of the spindle begin to grow usually 

 from the poles of each of the radiations. The larger portion of 

 the radiation or astrosphere is formed by the elongated meshes 

 of the cytoplasm. The sector of the radiation known as the 

 spindle seems to be formed by the substance of the plastosomes, 

 (M. Lewis, Robertson), or archoplasm (McClung), and consists 

 of uniform thin threads (fig. 26). It is transported to the poles 

 of the nucleus by currents, which again transform the cytoplasm 

 into radiations appearing this time as bipolar. The cytoplasm 

 becomes denser at the poles of the nucleus where the radii of the 

 radiation converge. In these regions appear the structures 

 known as centrosomes and centrioles. 



The poles of the nucleus during the growth of the spindle show 

 frequently a marked depression. The whole nucleus sometimes 

 may be compressed so energetically that it becomes oval in the 



