SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 253 



stains with Berlin blue ; in poorly preserved material the outlines of 

 the granules appear to form a reticulum. 



C. THE NUCLEOLUS 



A distinct plasmosome or true nucleolus comparable to the analo- 

 gous structure in Metazoa apparently exists in no case save possibly 

 in Actinospfuzrium, and even here it is limited to a passing phase during 

 mitosis (Hertwig, '98). It is probable that the structures which have 

 been almost universally but erroneously called nucleoli, do not belong 

 at all to this category of nuclear elements, but represent either the 

 functional chromatin which is aggregated into a central mass (karyo- 

 some) during the quiescent or vegetative period of cell-life, or the intra- 

 nuclear division centre. 



D. THE CHROMATIN 



The form which the chromatin assumes in Protozoa gives rise to a 

 great variety of nuclei which, at first sight, appear to have little in 

 common with each other or with nuclei of tissue-cells. There is, 

 however, a certain relationship between the different types, from 

 extremely simple conditions, to structures as complex as in Metazoa, 

 and through them there is a possibility of ultimately explaining the 

 chromatin-changes in Metazoa. 



Five types of nuclei, based upon the disposition of the chromatin, 

 can be distinguished. Of these the most primitive is, (i) the solid 

 sphere or karyosome (Binnenkorper Rhumbler), which has neither linin 

 reticulum nor membrane (e.g. Calcituba}. An advance is shown in 

 (2) nuclei having one such karyosome surrounded by karyolymph, the 

 whole inclosed within a membrane (vesicular nuclei, Gruber, '84), 

 while still higher types are : (3) nuclei with several karyosomes (two 

 to thirteen or fourteen), with membrane, karyolymph, and with or 

 without a nuclear reticulum (e.g. Noctiluca) ; (4) nuclei with a large 

 number of smaller masses of chromatin inclosed in a definite mem- 

 brane with or without a linin reticulum (e.g. Amceba proteus) ; (5) nuclei 

 consisting of granules of chromatin unconfined by a nuclear membrane 

 and spread over the entire cell (distributed nucleus), or aggregated 

 about a central body (" intermediate " type; Calkins, '98; e.g. 

 Tetramitus). 



Many Phytoflagellida, Choanoflagellida, some Sarcodina, and sporo- 

 zoites, among the Sporozoa, have chromatin in the form of a single 

 homogeneous karyosome. They are usually very minute, and little is 

 known about them save that they are undifferentiated, and that they 

 divide by a simple constriction into two equal parts. 



A transition to the second type of nucleus is shown in Calcitiiba 



