8 BE1TISH FRESHWATER RHIZOPODA. 



brane and a single compact mass of chromatin, which 

 occupies the centre of the distinct nucleus and is separ- 

 ated from the membrane by hyaline matter. In others 

 the chromatin reservoirs may be two or more, or there 

 may be a great number of granules in the nucleus with- 

 out the reservoirs (e. (j. Amoeba proteus). " In some of 

 the Rhizopoda (Euglypha) and Heliozoa (Actinophrys 

 and Actinospherium), the nucleus is strikingly similar 

 to that of metazoan cells, consisting of chromatin in the 

 form of a reticulum, and a network of linin." 



A true nucleus, comparable to the nuclear structure 

 in Metazoa, according to Hertwig, apparently exists in 

 no case, save possibly in Actinosplierium, and even here 

 it is limited to a passing phase during cell-division. 

 Discussing this subject Calkins remarks on the prob- 

 ability 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 (karyosome} during the 

 quiescent or vegetative period of cell-life, or the intra- 

 nuclear division-centre. 



This author thus summarises the observations of 

 Gruber, Hertwig, Brauer, and others upon the pheno- 

 mena presented by the nucleus, during mitosis (cell- 

 division), and concludes : " The facts point toward the 

 conclusion that the centre of activity in the division of 

 the protozoan cell, as in Metazoa, resides in a special 

 structure, which, to avoid confusion in terminology, 

 has been called the division-centre. In some cases this 

 structure resembles the astral system of Metozoa, in 

 consisting of an outer spherical mass with radiating 

 processes (astrosphere), and an inner focal granule or 

 granules (centrosome) . The evidence further tends to 

 show that the division-centre in Protozoa consists of 

 a specific substance different from the chromatin and 

 from the cytoplasm, and possessing above all other 

 portions of the cell an active role in division. No con- 



* Calkins, op. cit., p. 87. 



