PROTOZOAN NUCLEI. 398 



record on the structure of micro- and macronuclei, but it is 

 possible that the facts giv^en above may throw some hght on 

 their origin. A number of theories have been advanced to ex- 

 plain the origin of the micronucleus and the aberrant type of 

 nuclei in the Infusoria in general. The usual form of theory is 

 that the two types gradually arose by differentiation of a prim- 

 itive bi-nucleated form, one of the nuclei becoming the micro- 

 nucleus, the other the macronucleus (Butschli, Lauterborn, etc). 

 A serious objection to this theory is that the macronucleus is 

 formed from one of the subdivisions of the micronucleus at 

 each conjugation. Schaudinn suggested in his paper on Para- 

 moeba ('96) that the micronucleus and macronucleus of the In- 

 fusoria might have arisen from the Nebenkorpcr and nucleus 

 respectively of forms like Paramoeba. The possession of chro- 

 matin by the micronucleus is a serious obstacle to this theory, 

 and yet the important part which the micronucleus plays in re- 

 production makes Schaudinn's suggestion valuable. If pure 

 hypothesis be allowed it might be conceived that the micronu- 

 cleus represents the cytoplasmic body of forms like Tctramitiis 

 and Parmnoeba plus a certain amount of chromatin while the 

 macronucleus represents the nucleus with the remnants of chro- 

 matin minus the essential cytoplasmic body. The cytoplasmic 

 body which appears to be essential to reproduction as shown 

 by its universal presence, is found, in most cases, in only one 

 of the nuclei, which persists, while the other degenerates. 



General Co>xlusion'. 



Enough has been given above, I believe, to show that a type- 

 form of nucleus can be found to which the nuclei of the various 

 groups of Protozoa can be compared ; divergent forms being 

 explained as modifications of this type. Such a nucleus can 

 be described in brief as consisting of two distinct substances, 

 one of which acts as an "attraction" center, the other as chro- 

 matin in the form of granules. From this primitive type two 

 lines seem to have developed, in one of which the attraction cen- 

 ter remains outside of the nucleus {Noctiluca, Paramcebd) while 



