740 CALKINS. [Vol. XV. 



('95), and others find similar results in the Sporozoa. Similarly 

 Schultze ('66) described the nucleus of Givviia as variable ; one 

 type containing very large granules ; another, granules of 

 medium size ; and still a third type with very fine granules. 

 Other observers on different forms of Protozoa have given 

 similar descriptions. 



From what we now know about the chromatin changes in 

 Protozoa, it is probable that the different types of nuclei which, 

 like Gromia, have been described for the same organism are, 

 in reality, developmental stages in preparation for division. 

 Gruber ('83) first showed that in Actinospliaei'ium disintegration 

 of the central chromatin-mass is the earliest indication of 

 mitosis. He found that it first divides into two portions, then 

 into four, later into eight, and so on until a great number of 

 minute chromatin elements fills the nucleus. The nuclear 

 plate is then formed, and division of the nucleus ensues. This 

 observation was confirmed by Hertwig ('84); and Brauer ('95) 

 gave the same general result.^ 



In other Protozoa the chromatin is permanently in the form 

 of small chromatin elements. This is the case in Amoeba 

 pro tens, Amoeba biimcleata, and Amoeba ciystalligera, Euglcna 

 viridis, Ciyptomonas, and the macronuclei of the Infusoria. 

 From Schaudinn's description ('94) the chromatin elements in 

 Amoeba crystalligera do not change during mitosis but are 

 simply separated into equal parts by nuclear division. Similar 

 results were obtained by Blockmann ('94) and Keuten ('95) in 

 Eiiglena viridis. 



According to the latter's account, the nucleus of this flagellate consists, 

 as in Amoeba crystalligera, of a peripheral ring of elongate chromatin 

 bodies, and a central " nucleolus." In division the nucleolus first elongates 

 while the chromatin is arranged in radial lines. Elongation of the nucleolus 

 continues until the connecting-piece is reduced to a thin fiber. The chro- 

 matin is not formed into chromosomes, but is divided into two equal masses, 



1 Brauer's account of the resting nucleus differs in detail from that of Hertwig 

 and Gniber. He showed the nucleus to be similar to that of a metazoan cell, con- 

 sisting of chromatin in the form of a reticulum and " achromatin " in the form of linin. 

 1 have e.xamined many hundred nuclei of Actinosphaerium in the vegetative state 

 and after fixation with sublimate acetic and Hermann's fluid, but in none of 

 them could I find the mononucleolate nucleus described by Gruber and Hertwig. 

 The chromatin reticulum was invariably present with from two to four net knots. 



