CELL-ORGANS OF SEA-URCHIN EGG 573 



stance seems to be attracted by the apparently achromatic nu- 

 cleus, but in its movement encounters a great obstacle in simi- 

 lar strong currents directed from other chromatic accumula- 

 tions. As result of these processes, many small cytasters appear. 

 If they are not numerous, they gradually disappear, because of 

 the flowing of the substance to the nuclear region. 



No doubt many cytasters contain a basophilic and intensely 

 staining corpuscle in their center. These corpuscles sometimes 

 appear as verj^ distinct granules; again they are sometimes dif- 

 fusely stained, and do not show precise limits. Some of the 

 cytasters do not contain any such central bodies, yet there is no 

 other distinction between the different kinds of cytasters. 



The appearance of artificial cytasters may be accounted for 

 by the existence of physico-chemical processes similar to those 

 found in normal fertilization, and seems to depend upon a more 

 vigorous action of the same forces. The study of artificial cy- 

 tasters invariably indicates that a radial arrangement of the 

 cytoplasmic meshwork appears, when currents are present, 

 which transport certain substances from one part of the cyto- 

 plasm to the other. A radial arrangement of the cytoplasm 

 appears when the basophilic substance is transported toward 

 the nuclear region. This radiation has the nucleus as its center 

 and is formed by a centripetal current. More or less well pro- 

 nounced radiations may be formed by centrifugal currents and 

 result from the dissolution of the basophilic accumulations. All 

 these radiations are temporary re-arrangements of a common 

 structural basis of the cytoplasm, become less pronouned and 

 finally gradually disappear, when the currents exist no more. 



A very interesting modification in the distribution of the baso- 

 philic chromatic substance is brought about by a special anomaly 

 in the nucleus, occasionally observed in the very early stages of 

 parthenogenetic development. Sometimes the action of the 

 chemical substances applied manifests itself so intensely, that 

 the achromatic part of the nucleus breaks into two, three, four 

 and more achromatic regions, which again assume a spherical 

 form. Sometimes these nuclear parts remain close together, 

 (fig. 22), in other cases they appear separated (figs. 5 and 20). 



