42 PROTOZOOLOGY 



When viewed in life, the nucleoplasm is ordinarily homogeneous 

 and structureless. But, upon fixation, there appear invariably achro- 

 matic strands or networks which seem to connect the endosome and 

 the nuclear membrane (Fig. 2, b, d). Some investigators hold that 

 these strands or networks exist naturally in life, but due to the simi- 

 larity of refractive indices of the strands and of the nucleoplasm, 

 they are not visible and that, when fixed, they become readily recog- 

 nizable because of a change in these indices. In some nuclei, however, 

 certain strands have been observed in life, as for example in the 

 nucleus of the species of Barbulanympha (Fig. 174, c), according to 

 Cleveland and his associates (1934). Others maintain that the achro- 

 matic structures prominent in fixed vesicular nuclei are mere arti- 

 facts brought about by fixation and do not exist in life and that the 

 nucleoplasm is a homogeneous liquid matrix of the nucleus in which 

 the chromatin is usually distributed as small granules. Frequently 

 larger granules of various sizes and forms may occur along the inner 

 surface of the nuclear membrane. These so-called peripheral granules 

 that occur in Amoeba, Entamoeba, Pelomyxa, etc., are apparently 

 not chromatinic (Fig. 2, a, e). The vesicular nucleus is most com- 

 monly present in various orders of Sarcodina and Mastigophora. 



The compact nucleus (Fig. 2, g-j), on the other hand, contains a 

 large amount of chromatin substance and a comparatively small 

 amount of nucleoplasm, and is thus massive. The macronucleus of 

 the Ciliophora is almost always of this kind. The variety of forms 

 of the compact nuclei is indeed remarkable. It may be spherical, 

 ovate, cylindrical, club-shaped, band-form, moniliform, horseshoe- 

 form, filamentous, or dendritic. The nuclear membrane is always 

 distinct, and the chromatin substance is usually of spheroidal form, 

 varying in size among different species and often even in the same 

 species. In the majority of species, the chromatin granules are small 

 and compact (Fig. 2, h, i), though in some forms, such as Nyctotheru-s 

 ovalis (Fig. 3), they may reach 20/x or more in diameter in some indi- 

 viduals and while the smaller chromatin granules seem to be homo- 

 geneous, larger forms contain alveoli of different sizes in which 

 smaller chromatin granules are suspended (Kudo, 1936). 



Precise knowledge of chromatin (thymo- or desoxyribose-nucleic 

 acid) is still lacking. At present the determination of the chromatin 

 depends upon the following tests: (1) artificial digestion which does 

 not destroy this substance, while non-chromatinic parts of the nu- 

 cleus are completely dissolved; (2) acidified methyl green which 

 stains the chromatin bright green; (3) 10 per cent sodium chloride 

 solution which dissolves, or causes swelling of, chromatin granules, 



