NUCLEI AND KINETIC ELEMENTS 



57 



able that some functions are possible by virtue of the physical prop- 

 erties of a definite, but permeable, nuclear membrane, and as in the 

 tissue cells of IVIetazoa, it is this type of membrane-bound nucleus 

 that we find in the vast majority of Protozoa. 



In their resting stages the nuclei of Protozoa present a bewildering 

 variety of forms and structures, differing in this respect from the 



C 







Fig. 22. — Types of vesicular and massive nuclei. A, vesicular type of Pelomyxa 

 hinucleata; B, of Polystomella crispa; both with multiple endosomes; C, nucleus of 

 Actinosphoeruim eichhornii with granular plastin {p); D, E, F, macro- and micro- 

 nuclei of Paramecium caudafum, the latter in different stages of vegetative mitosis. 

 (^4, B, after Doflein; C, after Hertwig; D, E and F., original.) 



much less variable tissue nuclei of the Metazoa. Because of these 

 manifold differences students of the Protozoa have experienced great 

 difficulty in grouping nuclei for purposes of description. They 

 agree, however, in recognizing two primary nuclear types, the 

 vesicular and the massive. Nuclei of the massive type more clearly 

 resemble the nuclei of spermatozoa being filled with small chromatin 



