62 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



In Amoeba proteus , dense nucleoli are numerous, usually appear- 

 ing peripherally, close to the honeycomb cortex. Within the 

 diffuse filamentous matrix of the resting nucleus, strikingly well- 

 defined helical fibrils have been demonstrated by Pappas (1956a, 

 1959), Mercer (1959), Manni (cited in Lehmann, 1958b), and Roth, 

 Obetz, and Daniels (1960). These appear usually in stellate 

 clusters, suggesting that individual filaments radiate from a 

 common point of attachment, or as strings of clusters as in Fig. 21 , 

 PL V. Each individual helix is about 250 m/x in length and 

 includes up to nine or so coils with a diameter of 30 m/x, the fibril 

 itself being 8 to 9 m/x thick. 



Roth, Obetz, and Daniels (1960) and Roth (unpublished) have 

 examined a series of stages in mitosis in A. proteus, extending 

 earlier observations of Cohen (1957). In prophase, the nucleus 

 assumes a spherical shape, nucleoli migrate medially and subse- 

 quently disappear, and the honeycomb layer of the nuclear cortex 

 vanishes. No helices are to be seen now within the nucleus, but 

 an irregular, heavy network of dense material assumed to be 

 chromatin appears. At metaphase, sizable gaps are apparent in 

 the nuclear envelope. Tiny dense chromosomes, 0-5 to 0-8 /x long 

 are arranged on an equatorial plate, and spindle fibers have 

 appeared. These have the appearance of slightly sinuous tubules, 

 about 15 m/x in diameter, and are arranged in irregular strings or 

 clusters embedded in an amorphous material (Figs. 20 and 22, 

 PI. V). In anaphase, spindle fibers similarly are visible both 

 between the separating chromosome groups and extending from 

 them toward the ends of the spindle (Fig. 23, PL VI). In late 

 anaphase, the nucleus is a very thin, cup-shaped body bounded by 

 the double envelope which now is complete. The chromatin is 

 less condensed than in metaphase. In telophase, chromatin occurs 

 as a diffuse network ; nucleoli are reappearing but the honeycomb 

 cortex still is absent. Following cell division, a reconstruction 

 phase occurs during which the honeycomb layer reappears, 

 coincident with a remarkable elaboration of what seems to be 

 nucleolar material (Fig. 24, PL VI). Extensive, somewhat con- 

 voluted sheets of this material connect corpuscular nucleoli in a 

 wide peripheral zone of the nucleus ; only occasional bits of less 

 dense material identified as chromatin still are visible associated 



