CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 361 



et al., by the method of microdissection. Kite ('13) found by this 

 method that the Hving cytoplasm of the egg of Asterias is an 

 apparently homogeneous and a very viscous gel in which micro- 

 somes and globules are suspended. This gel he found to be 

 very elastic so that when portions of it were drawn out with a 

 needle they would at once retract when released. On the other 

 hand he found that the nuclear substance of this egg, with the 

 exception of the nucleolus which is a quite rigid gel, is all in the 

 sol state. In the male germ cells (probably spermatocytes) of 

 insects he found that the cytoplasm and nucleus of the resting 

 cells ''are far too rigid to flow or change shape under such ex- 

 perimental treatment. In the dividing cells the spindle fiber is 

 an elastic concentrated thread of nuclear gel and its absorptive 

 power and refractive index are also different from those of the 

 dilute gel in which the spindle fiber is imbedded and from 

 which it cannot be entirely freed. The homogeneous gel in 

 which a telophase spindle is imbedded is so rigid that all the 

 surrounding cytoplasm can be cut away and the spindle and chro- 

 mosomes show no appreciable change ; metaphase, anaphase and 

 telophase spindles can be cut to pieces in Ringer's fluid and the 

 pieces are so rigid that they undergo no change in shape." Fi- 

 nally he concludes ''that cell division results primarily from con- 

 comitant shrinking and swelling or changes in water holding 

 power of different portions of the cell protoplasm. Many of 

 the structural elements of the mitotic figure separate out of the 

 protoplasm and change in rigidity according to their water con- 

 tent. During the prophase the nuclear substance becomes so 

 soft that movement of the components of the nucleus is affected 

 by flowing of the nuclear gel. The mechanism at the basis of 

 this flowing seems to be a change in the water holding power of 

 the nuclear components." 



Chambers ('15) found that "the dissection of the germ cells 

 of insects and of the frog reveals an extreme variability in con- 

 sistency of their protoplasm, depending probably upon their 

 water content." He also found that "in many egg cells and free 

 living unicellular organisms the surface layer of protoplasm may 

 be decidedly more rigid than the interior." The nucleus of a 



