CELL DIVISION IN EGGS OF CREPIDULA. 541 



solutions which are allowed to act for a correspondingly shorter time, all forms 

 of division are permanently suppressed, so that when eggs subjected to such 

 solutions are returned to normal sea water they never show any signs of division 

 either of cytoplasm, nucleus or centrosome. Nevertheless such eggs may 

 remain alive for several days and their nuclear vesicles may grow to enormous 

 size. In such eggs centrosomes and spheres are usually entirely lacking, or at 

 best are small and indistinct; the chromatin, which is always contracted into a 

 nucleolus-like mass within the nuclear vesicle while the eggs are in these solutions, 

 remains permanently in this condition when the eggs are returned to normal 

 sea water, and never again assumes a reticular form, nor gives rise to chromosomes. 

 I attribute the complete cessation of divisional activity to this inability of 

 centrosomes and nuclei to recover their normal forms and functions. 



The nuclear vesicles grow to a great size and in this condition they have all 

 the appearances of germinal vesicles of immature eggs. One cannot fail to be 

 impressed with the resemblance of such cells to immature egg cells, not merely 

 in their form but also in the loss of the power of division, their low metabolic 

 activity, and their ability to remain alive almost indefinitely while in this 

 condition. It would be worth while to treat such eggs with some of the various 

 reagents by which it has been found possible to stimulate immature eggs to form 

 polar bodies, in order to see whether divisional activity might again be revived 

 in them. 



In addition to the limiting action of hypertonic solutions on the division of 

 the cell body, the nucleus, and the centrosome, other striking results are produced 

 which will now be described. 



4. Shrinkage of Plasma, Nuclei and Mitotic Figures in Hypertonic Solutions. 



The first effect of hypertonic solutions on the eggs of Crepidula is to cause a 

 shrinkage, or contraction, of the plasma, nuclei, and mitotic figures, probably 

 accompanied by the loss of a small amount of water from the egg, as Loeb has 

 maintained. However since the diameter of the egg as a whole decreases very 

 little, the loss of water must be slight. Indeed this shrinkage shows itself not so 

 much in the decrease in size of the entire egg as in the more complete segregation 

 of the plasma from the yolk, on the one hand, and from the more fluid inclusions 

 (oil, water, etc.) on the, other. This is accomplished, apparently, by the with- 

 drawal of outlying portions and radiations of plasma into a central mass imme- 

 diately surrounding the nucleus, while at the same time the yolk and more fluid 

 inclusions are forced to the periphery of this mass. In this way a segregation 

 of these three constituents of the egg is accomplished, similar in the nature of the 

 substances segregated, but not in their orientation, to the segregation accom- 

 plished by centrifugal force. 



In wholly similar manner the shrinkage of the nucleus as a whole is relatively 

 slight, whereas the segregation of chromatin from achromatin is the most dis- 

 tinctive action of hypertonic solutions on resting nuclei. The chromatin within 



