88 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Later the " Nucleolen " break down and contribute their substance to 

 the formation of the chromosomes. " Die Nucleolen fangen an sich 

 heller zu farben, wahrend sich auf den achromatischen Geriiststrangen 

 Chromatintheile in feinster, unregelmassige Form vertheilen." They 

 become vacuolated and finally appear in section " als etwas unregelmas- 

 sige chromatische Ringe." At this time " D.is Chromatin ist in aller- 

 feinsten Flocken auf dem ganzen achromatischen Kerngertlst vertheilt." 

 Tlie chromosomes are later formed from these irregular granular masses. 



In the egg of one of the lower chordates (Distaplia), Davidoff ('89) has 

 observed that the large nucleolus increases in size by the addition of the 

 greater part of the nuclear reticulum, and that this material is later 

 used in the formation of the chromosomes of the polar spindle. 



Schneider ('91) asserts that in the eggs of Echinoderms the nucleoli 

 are only reserve masses of metamorphosed chromatin. That this is true 

 in some cases at least is shown by the recent observations of Wilson 

 (:01 a ) upon the eggs of Toxopneustes artificially fertilized with a solution 

 of magnesium chloride. He finds that in one series of the eggs thus 

 treated the chromosomes taking part, in mitosis are obtained by the 

 breaking down of the large deeply staining nucleolus. " Its contour be- 

 comes irregular and its texture loose. A little later it assumes a spongy 

 appearance and short irregular processes are extended from its pe- 

 riphery. Enlarging still more, it now gives almost the appearance of a 

 close, broken spireme, from the ends of which chromatin threads here 

 and there project." In the other series the chromatin is retained in the 

 form of a reticulum, as in the normal eggs, and the chromosomes are de- 

 rived from this reticulum. In attempting to explain this variation he 

 says : "I think that in one case the chromatin is gradually drawn from 

 the linin network through which it is originally distributed, In the 

 second case such a withdrawal does not take place, though the achro- 

 matic nucleolar material accumulates as before." These conclusions of 

 Wilson confirm the earlier observations of Hertwig ('96), who ob- 

 tained similar results upon Echinoderm eggs treated with strychnine. 



In Limnaea Linville (:00) also reports observations which he thinks 

 indicate a close connection between the nucleoli or karyosomes and the 

 chromatin elements. He however has made no detailed study of the 

 chromatic structures, and therefore hesitates to assert definitely that 

 the chromosomes are of nucleolar oriain. 



From the preceding brief review it will be seen that in Protozoa the 

 aggregation of the chromatin into karyosomes or more complex karyo- 

 spheres is of general occurrence, while the presence of the spireme or 



