76 THE PROTOZOA 



linin framework as different in any essential point from the 

 alveolar framework of the general protoplasm, with which, in nuclei 

 that lack a true membrane, it is perfectly continuous. Hertwig (66) 

 regards the cytoplasmic framework as achromatinic substance in 

 intimate combination with chromatin ; the nuclear framework, on 

 the other hand, as pure achromatinic substance (linin) from which 

 the chromatin has become separated out and organized into special 

 structures independent of the framework in which they are lodged. 

 Similarly, the nuclear sap filling the nuclear space and the inter- 

 stices of the reticulum must be identified with the enchylema of 

 the body-protoplasm. As compared with, the alveolar structure 

 of the general protoplasm, that of the achromatinic nuclear frame- 

 work is characterized chiefly by the larger size of the alveoli, and, 

 consequently, the greater distinctness of the apparent reticular 

 structure. 



A true nuclear membrane, when present, is probably formed in 

 all cases from the achromatinic framework. In the nuclei of Actino- 

 sph cerium, according to Hertwig (64), the membrane is a super- 

 ficial condensation of the achromatinic reticulum. The membrane 

 may attain to a considerable thickness and appear doubly-con- 

 toured in optical section, separating the nuclear framework com- 

 pletely from the extranuclear protoplasm ; but it is always a structure 

 very readily absorbed and re-formed, and it appears to present 

 no obstacle to the passage of substance from the nucleus into the 

 cytoplasm, or vice versa. Awerinzew (47), on the other hand, 

 regards the nuclear membrane as a product of the cytoplasm. 



In addition to the achromatinic framework, plastin is commonly, if 

 not invariably, present in the form of masses or bodies which receive 

 different names, according as they consist of pure plastin or of 

 plastin impregnated to a greater or less extent with chromatin. 

 In the vesicular type of nucleus, the endosome may perhaps consist, 

 in some cases sf pure chromatin, but in most cases, if not always, 

 it is composed of a matrix or ground-substance of plastin in which 

 the chromatin is lodged. An endosome of this kind is termed a 

 karyosome, or chromatin-nucleolus ; as a rule it has the form of a 

 rounded mass, occupying the centre of the nucleus, sometimes of 

 more than one such mass, but in a few cases it may have the form 

 of a crescent or cap (" calotte ") closely applied to the nuclear mem- 

 brane. In the granular type of nucleus, on the other hand, there 

 may be one or more masses of pure plastin containing no chromatin ; 

 such a body is termed a nucleolus simply, or a " plastin-nucleolus." 

 In tho nuclei of the tissue-cells of Metazoa, true nucleoli occur 

 almost invariably ; in the nuclei of Protozoa, however, pure plastin- 

 nucleoli are not of common occurrence, but have been described 

 in a few instances for example, in the hsemogregarine-nucleus 



