SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 



which, as in some Metazoa, splits lengthwise before it is broken into 

 chromosomes (Fig. 23, p. 55). A distinct spireme has also been de- 

 scribed in micronuclei of the Infusoria by Biitschli, Pfitzner, Maupas, 

 Hoyer, and Bergh, and in Heliozoa by Schaudinn, while some macro- 

 nuclei, as in Stylonychia and Loxophyllum, are apparently in a perma- 

 nent spireme stage (Fig. 104). The latter case is particularly 

 interesting, for, as in the nuclei from the salivary glands of the Chi- 

 ronomus larva (Balbiani), the spireme consists of alternating disks of 

 chromatin and a non-staining substance which Balbiani ('90) consid- 

 ered achromatin with the characteristics of plasmosomes. 



In Actinospk&rium and in Spirochona, Kentrochona, etc., the mass 

 of chromatin breaks down into granules which collect in lines across 

 the nucleus to form primitive chromosomes (Fig. 139). 



Noctiluca is apparently about midway in complexity between 

 Actinosphcerium and Euglypha. The ten or eleven karyosomes, as in 

 Sporozoa and Rhizopoda, break down into an immense number of 

 minute chromatin granules, which collect, at first, in groups in the 

 region of the reservoirs from which they were derived (Fig. 141); 

 but they are later distributed about the nucleus. The granules are 

 then collected in lines which radiate inward from one side of the 

 nucleus. By the constant addition of granules, thick chromosomes are 

 formed, which split down the centre. Again there is no evidence to 

 indicate division of the granules or the presence of a definite spireme 

 stage. 1 



From the foregoing review, the facts at present appear to indicate 

 that the most primitive nucleus is the single mass of chromatin with- 

 out membrane or reticulum (Coccidiida, Reticulariida). The simplest 

 nuclear membranes are formed directly from this chromatin (Schau- 

 dinn in Calcituba\ as is one type at least of the nuclear reticulum 

 (Reticulariida and Sporozoa). The primitive nuclear mass breaks down 

 into granules in preparation for reproduction, a phenomenon which is 

 almost universal in Metazoa and Protozoa. In the latter group, the es- 

 sential feature, apparently, is the division of the chromatin mass, rather 

 than of the chromatin granules, as seen by the collection of chromatin 

 granules of distributed types (e.g. Tetramitus) into one group which 

 is halved, or in the formation of primitive chromosomes as in Actino- 

 sphcerium or Noctihica. The division of the chromatin granule is 

 apparently not necessary, as shown by the thick chromatin aggre- 

 gates in Noctiluca and Actinosph&rium, and in the peculiar relations 

 in some Ciliata (Urostyla), where the long lines of chromatin are 

 divided into separate segments, each of which forms a nucleus. 



1 Doflein ('oo) has recently given a very different account of chromosome-formation in 

 Noctiluca, based upon incomplete observations. 





