THE NUCLEI OF UROLEPTUS MOBILIS 343 



reducing division if the conditions in protozoa are the same as in 

 metazoa. This, however, cannot be granted offhand for the 

 second division is also longitudinal, and it was impossible to tell 

 whether the double chromosomes were likewise longitudinally 

 divided. 



Waldeyer ('88) defines chromosomes as "the deeply staining 

 bodies into which the chromatic nuclear net-work resolves itself 

 during mitotic cell-division" (quoted from Wilson, '99). Apart 

 from the chromatic nuclear net-work from which they originate, 

 this definition would include the chromosomes of the cihate 

 micronucleus. But it would also include the granules of chro- 

 matin which elongate and divide in the typical macronucleus 

 during division. The number of chromosomes, furthermore, is 

 regarded as fixed for the same species, but the number of dividing 

 chromatin granules in the macronucleus is immeasurably greater 

 than the number of chromosomes of the micronucleus. It follows 

 that either these dividing granules of chromatin are not all chro- 

 mosomes or that the number of chromosomes is not fixed for the 

 same species in all cases. It is obvious that, according to the 

 definition, all chromatin bodies which arise from the chromatin 

 of the resting nucleus are not chromosomes. The difficulty in 

 most cases might be avoided by a strict interpretation of the 

 definition which involves 'mitotic' cell division, although in some 

 cases, e.g., Spirochona gemmipara (Hertwig, '77), the macro- 

 nucleus divides by mitosis. The suggestion made by Rabl that 

 the chromosomes retain their individuality throughout life of the 

 organism has grown to a strong conviction in later years, es- 

 pecially through the work of Conkhn, Morgan and recently 

 Richards. I doubt very much, however, if anyone can be found 

 hardy enough to apply this conception to the chromosomes of 

 protozoa. Nevertheless, I believe that we are justified in re- 

 garding chromosomes in ciliates as equivalent structures to 

 chromosomes of the metazoa. In the prophases of the first 

 maturation spindle in ciliates we find parallel stages with those 

 of the first maturation division in higher animals. The synezesis 

 stage is represented by the chromatic net- work which arises from 

 the homogeneous chromatin in the parachute type of nucleus 



