64 KARYOKINESIS. 



are such is beyond dispute. Certainly, structures which function as centrosomes 

 through a long period leading up to the production of larvae (Loeb, Wilson), are 

 enough like centrosomes to pass under that name. And even in cases where larvae 

 are not produced (experiments of Hertwig, Mead and Morgan), there can be no 

 reasonable doubt that centrosomes are found in the larger asters, even if the smaller 

 ones do not contain them. 



In the case of Morgan's experiments on fertilized eggs it might be maintained 

 that the numerous asters and centrosomes observed are derived by division or frag- 

 mentation from those already present in the cell-body, where it not for the fact 

 that similar asters and centrosomes have been observed in the case of unfertilized 

 eggs (Hertwig, Morgan, Wilson) where no centrosomes are present in the cytoplasm. 

 The phenomena in these two cases are so similar that one cannot believe that they 

 are due to wholly different causes ; we may, therefore, safely class them together. 



Hertwig maintains that in his experiments the centrosomes were formed from 

 the achromatic constituents of the nucleus. He says: " Ich deute somit die Cen- 

 trosomen as selbstandige gewordene geformte achromatische Kernsubstanz, eine 

 Deutung fur die ich wiederholt eingetreten bin." In part Morgan agrees with this 

 position, though he also holds that centrosomes may arise at a distance from the 

 nucleus and therefore from the cytoplasm. "I agree," he says, "with Hertwig 

 that the centrosomes may develop out of the achromatic substance of the nucleus, 

 but I see no ground to extend this statement to include all centrosomes. . . . There 

 is good evidence to show, I think, that similar bodies may arise in the cytoplasm 

 also, as shown by Reinke, Mead, Watase and myself." It is a notable fact, how- 

 ever, that in all these cases cited by Morgan the nuclear membrane has disappeared 

 or is much shrunken and collapsed, showing that nuclear substance has escaped 

 from it. This is true at least of the figures of Reinke, Mead and Morgan ; Watase 

 gives no figures of the egg of Macrobdella which he describes as containing " a 

 series of thirteen asters, ranging from the miniature aster, with the microsome in 

 its center, to the normal aster with a veritable centrosome." In the figures of 

 Reinke, Mead and Morgan one is much struck by the fact that at the time when 

 the asters appear in the cell the nuclear membrane is either entirely lacking or is 

 much shrunken, showing that achromatic material has escaped into the cell. Of 

 his own experiments Morgan says (p. 464) : " The first effect (of the salt solution on 

 the egg) is to cause a shrinkage of the nucleus ; then after the return of the eggs 

 to sea water the division of the nucleus and subsequently that of the egg takes 

 place ;. . . . central bodies are present in the artificial astrosphseres in almost all 

 the stages." Again (p. 517) he says: "At the time when the nuclear wall dis- 

 appears the astrospheeres throughout the egg, whether in contact with the chromo- 

 somes or not, become conspicuous and then fade away again as the chromosomes 

 pass into the resting nuclei. There is some connection between the setting free of 

 the chromosomes and the development of the astrospha?res ;" or rather, as it seems 

 to me, between the escape of some nuclear constituent and the development of the 

 astrosphaeres. The fact that achromatic nuclear substance maj^ be distributed 

 widely through the cell in normal mitoses was pointed out in the section on aster 



