CELL-ORGANS OF SEA-URCHIN EGG 587 



various as they are in normally developing eggs. If during re- 

 sorption of the cytoplasmic basophilic substance numerous small 

 chromatin particles are formed at the inner periphery of the 

 nuclear membrane, soon some of these chromatin particles be- 

 come larger, others free themselves from the membrane, connect 

 together by outgrowing filaments, and finally form a chromatin 

 net. If the resorption is more intense in some parts of the 

 nuclear surface, larger chromatin bodies are formed, which con- 

 tribute also to the development of the chromatin net (fig. 21). 

 It has already been mentioned that complex chemical inter- 

 changes must take place between the achromatic part of the 

 nucleus and the absorbed basophilic substance, the final results 

 of which are represented by the differentiation of chromosomes. 



Some interesting. changes are undergone by the archoplasm or 

 plastosomes in artificial parthenogenesis. There are frequent 

 cases in which the plastosomes show regressive changes, some- 

 times they are discovered in the cytoplasm, but not attracted 

 by the nucleus. In other cases they flow very uniformly towards 

 the whole periphery of the spherical egg and form here a single 

 radiation. In such cases no bipolar radiations arise, nor a 

 spindle. This is the case of a typical monaster without division 

 of the nucleus. The chromosomes are differentiated, but no 

 movement of the chromosomes follows. Another cycle of dif- 

 ferentiation of chromosomes begins (fig. 23), without division 

 of the nucleus. Evidently the division of the nucleus requires a 

 participation of plastosomes or archoplasm in the development 

 of the spindle. 



In some eggs the archoplasm accumulates irregularly around 

 the nucleus and from the centers of its accumulations grows 

 within the nucleus in the usual form of threads which are fas- 

 tened to the chromosomes. In such cases numerous centers 

 arise at once, to which the chromosomes are transported. The 

 partial movement of the chromosomes forms sometimes very 

 typical half spindles. 



Finally another interesting anomaly in the formation of the 

 spindle may be observed. The spindle is formed without any 

 precise radiation in the cytoplasm (fig. 25). In this case the 



