CHROMIDIA 11 



greater uncertainty. Some observers believe that the chromatin granules 

 or chromidia in the cytoplasm may, under certain conditions, arrange 

 themselves in groups, each of which becomes transformed into a nucleus. 

 It is difficult to avoid the impression that most, if not all, of the records 

 of nuclei arising, as it were, by crystallization of chromidia are the result 

 of misinterpretations, and that the appearances on which the conclusions 

 have been based might be accounted for in another and more probable 

 manner. In all cases in which accurate and continuous observation of 

 reproducing cells has been possible, daughter nuclei have been found to 

 arise only by division of pre-existing parent nuclei. A classical instance 

 of this kind is seen in Arcella vulgaris, a binucleate shelled amcBba 

 (Figs. 2 and 79). Like many other shelled amoebae, in addition to the 

 true nuclei, Arcella vulgaris contains a mass of material which, on account 

 of its affinity for certain chromatin stains, is supposed to be of chromidia! 

 nature, and is called the chromidial body. It was claimed by Richard 

 Hertwig (1899) and other observers that at certain phases of development 

 the two existing nuclei degenerate and disappear, and that numerous 

 secondary nuclei are formed from the chromidial body. Schirch (1914) has, 

 however, shown that in some cases, at least, the numerous nuclei which 

 are present result from repeated divisions of the two which occur in the 

 normal individual. It seems not improbable that the so-called chromidial 

 body of Arcella and its allies is not really of chromatin nature, but consists 

 of a special material which may be concerned with the development of 

 the shell, which is a characteristic feature of these shelled amoebae. 



TYPICAL DIVISION OF THE METAZOAN NUCLEUS. 



1. Mitotic Division. 



The Protozoan nuclei divide in a variety of ways, and it is probable that 

 amongst them the more primitive types of nuclear division will be found. 

 There is every transition between what is little more than a simple con- 

 striction of the nucleus into two parts {amitotic division) and the elaborate 

 method of division known as mitosis or haryokiyiesis, in which chromosomes 

 are formed and divided in such a manner that the chromatin of the nucleus 

 is equally distributed to the daughter nuclei. The division of nuclei by 

 mitosis occurs most typically in the cells of higher animals and plants, 

 and it was in their cells that the details of the process were first elucidated. 

 The terms employed for the different structures and the various stages 

 which occur were first applied to their nuclei, and were used subsequently 

 for the corresponding stages which occur during the division of Protozoan 

 nuclei. Mitosis in its typical form is characterized by the formation 

 from the chromatin and achromatic material of the nucleus of a number 



