208 INTRODUCTION TO CYTOLOGY 



chromatic substance is seen also in the protozoa. Here the chromatic 

 material in a number of species is held together in loose granular aggrega- 

 tions, and the achromatic figure is seen in curious, relatively simple 

 manifestations. In many cases, however, even among the Rhizopoda, 

 there occurs a very advanced type of mitosis, with spindle, centrosomes, 

 asters, and a definite number of chromosomes. 



One of the simplest types of nuclear division found among the pro- 

 tozoa is known as "chromidial fragmentation." Here the nucleus is 

 resolved into a large number of chromatin granules, or chromidia, which 

 reassemble in two or more groups and form new nuclei. In nuclei of the 

 " vesicular type," found commonly among the Microsporidia, the chroma- 

 tin is concentrated in a single large body, or karyosome, which becomes 

 dumbbell-shaped and divides, the rest of the nucleus then dividing also. 

 In other forms, such as Coccidium (Fig. 73, B), the nucleus contains in 

 addition a second kind of chromatin which is approximately halved in 

 nuclear division. It is but a short step from such non-mitotic division 

 as this to the simplest types of mitosis (" promitosis") seen in many 

 protozoa. Within the group are found all gradations in complexity 

 from such primitive modes of division up to the advanced types showing 

 a complete achromatic figure with chromosomes which are regular in 

 form, number, arrangement, and division, just as in the higher animals. 1 



Both Metcalf and Kofoid (1915) have emphasized the fundamental 

 similarity of protozoan and metazoan nuclei. The process of mitosis 

 has the same succession of phases in the two cases, though many minor 

 variations occur. In some representatives of all the main groups of 

 protozoa are found elongated chromosomes which Metcalf regards as 

 linear aggregates of chromatin granules, and which split longitudinally, 

 giving exact equivalence to the daughter-nuclei. Although the cell 

 mechanism of Mendelian inheritance is thus held to be present in members 

 of each great protozoan group and to operate as in the metazoa at the 

 sexual stages, Metcalf believes this mechanism is not kept intact through 

 the vegetative phases as it is in the higher groups. 



Other Cases in Plants. — With regard to the myxomycetes, the 

 researches of Strasburger (1884), Harper (1900), Jahn (1904, 1911), 

 Olive (1907), and Winge (1912) have shown that nuclear division is 

 essentially mitotic, and that in some cases the chromosomes are not 

 only definite in number but undergo a reduction prior to spore formation. 

 As an example of an exceptional condition may be taken Sorodiscus 

 (Fig. 74, A), in which Winge describes two sorts of chromatin: vegetative 

 tropho chromatin and generative idiochromatin, the two forming a single 

 mass at the center of the nucleus. As nuclear division begins this mass 

 takes the form of three or four bodies very similar to chromosomes. 



1 For a description of mitotic phenomena in protozoa see Minchin (1912, Chapter 

 VII). 



