THE FUNCTION OF THE CENTROSOME. 



215 



two blastomeres, while the lobe is developing, and the latter is 

 borne upon the larger of the two cells, into which it is after- 

 wards resorbed. On the other hand, in the unfertilized egg 

 stimulated by the salt, the lobe, though it is formed in exactly 

 the same manner, is resorbed into the undivided egg, which 

 then once more resumes the form of a sphere. Occasionally a 

 furrow cuts deeply into the egg at the animal pole and then 

 vanishes again, and eggs are found which have every appear- 

 ance of being in the 2-cell stage. 



In regard to the effect produced by the potassium chloride, 

 I wish to emphasize two points: first, it is of the nature of a 

 stimulus, compatible with the continuance of the normal devel- 

 opmental processes, and is not of the nature of a poison or an 

 irritant setting up irregular, abnormal, and inconstant changes; 

 second, the stimulus must be referred to the specific properties 

 of the salt and not to a change in the density of the water in 

 which the eggs are placed, (i) If the unfertilized eggs are 

 allowed to remain in the potassium chloride solution for only a 

 few minutes and are then returned to normal sea- water, . the 

 effect is the same as though they were left permanently in the 

 salt solution. Furthermore, eggs may be fertilized in the po- 

 tassium chloride solution and reared to free-swimming trocho- 

 phores without being returned to the normal sea-water. (2) If 

 sodium chloride is added to the sea-water instead of potassium 

 chloride, no apparent effect is produced upon the oocyte ; but, 

 if the potassium is added to this solution, or, if spermatozoa 

 be introduced, the polar globules will be formed in the usual 

 manner. Similarly, if the sea-water is diluted by the addition 

 of y$ volume distilled water, no effect is produced; but, when 

 the usual amount of potassium chloride is added, the mitosis is 

 immediately resumed. 



The behavior of the unfertilized eggs that have been stimu- 

 lated by the salt is interesting in that it shows how many of 

 the cytokinetic and karyokinetic changes, which are initiated 

 by the entrance of the sperm, are independent of the sperm- 

 nucleus, " sperm-centrosomes," and asters. In the fertilized 

 egg, while the polar globules are being formed, asters are devel- 

 oped about the sperm-centrosomes. These asters grow con- 



