PROTOZOAN NUCLEI. 395 



even in the higher tissues, as Flemming ('97) suggests, it is 

 apparently not an organ of primary importance, but an organ 

 which may be present in connection with cell-divisions, although 

 not necessary for it. 



There is also good evidence in this study of primitive nuclei 

 to show that the common type offers an explanation of the 

 changes which the constituents of the metazoan nucleus undergo 

 during and preparatory to a division. Stated briefly this idea 

 may be expressed as follows : (i) Before forming chromosomes 

 the chromatin material of the metazoan nucleus is distributed 

 in the nucleus in the form of minute chromatin granules, a stage 

 representing the ancestral condition which in flagellates and 

 lower plants is permanent ; (2) the chromatin granules (Brauer,, 

 '93 Ascaris) secondarily fuse to form distinct bodies — the 

 chromosomes — of definite form and number for each species ; 

 (3) the chromatin is in close connection with the kinetic center 

 (centrosome or centrosphere plus central spindle), to accom-. 

 plish this connection the nuclear membrane disappears (in most 

 Protozoa the attraction sphere is inside the nuclear membrane 

 and so in constant connection with the chromatin ; in other 

 forms of Protozoa where the attraction sphere is extra-nuclear 

 as in Noctilnca and Paranioeba the membrane disappears on the 

 side of the nucleus nearest the sphere — Noctilnca — or there is 

 no membrane at all — Paraviwbd). In all cases the chromatin 

 at the time of division is collected around or between the spindle 

 fibers, or in case of Protozoa, the attraction-sphere, possibly to 

 ensure a more perfect division of this important substance. 



Summary of Observations and Conclusions. 



1. Metazoan and protozoan nuclei cannot be strictly homolo- 

 gized, but it can be shown that an intermediate series of forms 

 connect them. 



2. The nuclei of Protozoa are not all of the same type and in 

 some forms they may possibly be absent. The simplest struc- 

 ture is the distributed nucleus, consisting of isolated chromatin 

 granules scattered about the cell. 



