No. 3.] THE PROTOZOA AND METAZOA. 743 



case of Opalina ranartim. Schewiakoff ('88) gives a somewhat 

 similar description of mitosis in Eiiglyplia alvcolata. 



Here the chromatin net^vork of the nucleus gradually becomes thicker, 

 especially in the region of the net knots, until a coarse spireme appears. 

 Threads composed of many small particles — the "chromatin granules " of 

 Plitzner — are finally formed from the spireme; these are ragged at first, but' 

 become homogeneous and smooth, after which they arrange themselves 

 around the periphery of the nucleus. The threads then shorten and become 

 bent into loops, the angle being turned towards the axis of the spindle. At 

 this stage each chromatin-loop is divided longitudinally and the daughter- 

 chromosomes separate. After the formation of a daughter-spireme the 

 nucleus returns to the reticular state. 



From this examination of the different changes undergone 

 by the chromatin in the various forms, it is possible to get an 

 idea of the probable development of chromosomes, although the 

 many gaps in the series and the often incomplete observations 

 make it far from conclusive. The most primitive structure 

 would seem to be the mononucleolate forms in which the 

 nucleus has no " cell sap," and where division is possibly 

 "amitotic." An advance is shown in forms where the mono- 

 nucleolate chromatin-mass breaks up into smaller elements 

 during division, as in some Rhizopods and Heliozoa, and in 

 many Protozoa just before spore-formation. A still higher 

 type is shown in forms where the chromatin exists permanently 

 in the form of small granules (many Amoebae, Euglena, etc.). 

 In Euglena and Amoeba these chromatin-granules do not fuse 

 to form definite structures (chromosomes) ; they simply separate 

 half from half, and they are clearly equivalent to the minute 

 elements, which in other cases are formed by the breaking 

 down of chromatin-masses. The aggregation of chromatin 

 elements into more or less definite chromosomes is shown in 

 a primitive way in Actitwsphaerium and the micronuclei of 

 Infusoria. In Noctiluca the aggregates become more compact, 

 definite, and chromosome-like, while for the first time they are 

 divided longitudinally. A still more metazoan-like chromosome 

 structure is shown by Euglypha alveolata, where the chromatin 

 elements are not distributed throughout the nucleus, but unite 

 at once to form a tightly wound thread — the spireme — from 

 which the chromosomes are later derived by transverse division. 



