354 SYDNEY J. HICKSON. 



chromatin and that divides by mitosis is a nucleus. It is 

 frequently very difficult, however, to distinguish true nuclear 

 chromatin from substances in the cytoplasm that are not 

 chromatin, and there are many examples of nuclei known to 

 science that do not divide by mitosis. It may be taken, 

 however, as a further axiom of histology that every structure 

 originating as a daughter nucleus by mitosis of a pre-existing 

 nucleus is itself a nucleus. 



Both the meganucleus and the micronucleus of the Hetero- 

 karyote body, therefore, ai*e true nuclei; the former on the 

 ground that it originates from the nucleus formed by the 

 mitotic division of a micronucleus, notwithstanding the fact 

 that it always divides amitotically,^ and the latter on the 

 ground that it divides by mitosis. These two nuclei, how- 

 ever, differ from each other in several important particulars. 

 The meganucleus is very much larger in bulk during the 

 somatic life of the individual than the micronucleus. In 

 fission or gemmation it divides amitotically. It does not 

 divide during conjugation, but during or at the close of this 

 process it ceases to exist as a definite entity. 



The micronucleus, on the other hand, is very much smaller 

 than the meganucleus dui-ing somatic life. In fission and 

 gemmation it divides by mitosis. It does divide, again by 

 mitosis, dui-ing conjugation, and one of the products of its 

 division gives rise to the germ-nuclei. It is not necessary to 

 discuss further in this place the relations of these two nuclei. 

 The reasons set forth by Biitschli with masterly ability in his 

 great work on the Infusoria, for considering the meganucleus 

 to be the "somatic" nucleus, and the micronucleus as the 

 " sexual " nucleus, are sufficient for my purpose. If, how- 

 ever, we accept the view that in the body of the Heterokaryote 

 there is one (or occasionally more than one) somatic nucleus 

 and one or more than one sexual nuclei, we are led to the 

 further inquiry whether there is also a distinction between 

 the somatic cytoplasm and the sexual cytoplasm. 



1 Apparent exceptions to tliis rule :ire afforded by tlie meganuclei of 

 Opalina and Kentrocliona. 



