88 BIOLOGY OF THE PROTOZOA 



bacteria-like organisms of non-cellular grade, in which the chroma- 

 tin is permanently distributed. Doflein (1916) remains skeptical 

 in regard to this type of free-nuclei formation and Kofoid (1921), 

 apparently without investigation of free-living forms, maintains 

 that such free nuclei are intracellular parasites. It is evident that 

 the burden of proof here rests with the critics. (See also p. 71.)' 



Fig. 45. — Chromidia formation in Mastigella and Mastigina. A, B, young forms 

 of Mastigella vitrca prior to chromidia formation; C, chromidia arising from the 

 nucleus; D, young form of Mastigina sctosa with accumulation of chromidia; E, F, 

 mature stages of M. setosa; G, formation of gametic nuclei (a) from scattered chro- 

 midia. (After Goldschmidt.) 



3. Nuclear Derivatives During Division.— The substances compos- 

 ing nuclei— karyolymph, plastin, chromatin and kinetic elements- 

 are apparently inert during vegetative life, inert at least so far as 

 demonstrable activity is concerned. Metabolic activities which 

 result in cell division, however, are manifested periodically by 

 characteristic changes in these substances, and structures not 

 present before — spindle elements and chromosomes — are formed 

 which, after a brief existence, pass again into the apparently inert 

 condition of the vegetative nucleus, that is, they are reversible. 

 Theoretically such transient phases are the most important of all 

 stages in the life history for they involve the formation and division 

 of chromosomes, which are regarded as the vehicle of hereditary 

 characteristics, and the kinetic elements, which are regarded as 

 instrumental in bringing about such formations and divisions. 



(a) Origin of Chromosomes and of Intranuclear Spindles at Divi- 

 sion. — The nucleus is the most complex of the formed organoids 

 of the cell, and its reproduction involves growth and division of 



