EXPLANATION OF PLATES. 



Plate I. 



Fig. 1. Area of new growth from Xo. 42, 8: 12: 14 (Lewis collection). In the field are six binueleate (a) and one 

 quadrinucleate (b) cells. The material was heart from a chick which had been incubated for 6 days; 

 growth was of 48 hours' duration, in Locke (0.5 per cent tlextrose), fixation by osmic-acid vapor, and 

 staining with iron hematoxj-lin. (0 shows two young daughter cells, the product of a recent mitosis. 

 The guide-line from (n) terminates in the centrosphere of a l)inucleate cell. Retouched photograph. 

 X46.5. 



Fig. 2. Elongated nucleus with bilateral constriction — the beginning of direct bilateral nuclear fi.ssion. The nucleolus 

 is also ap])arcntly dividing. This figure, and also Nos. 3, 4, 5, 11, 12, 13, 14, 15, 16, 17, 18, and 22 are 

 from No. 14, 9 : 1 ; 15 (Lewis). Heart from 6-day chick; grown in Locke (0.5 per cent dextrose) with a 

 little yolk; fixed on third daj- of growth in Zenker; stained with Mallory's connective tissue stain. On 

 account of the technique the cytoplasmic details are not represented. This and following drawings, 

 except figures 24 to 35, were outlined by camera lucida. X 1,012. 



Fig. 3. Elongated nucleus almost completely divided; final stage of direct bilateral nuclear fission. Xl,012. 



Fig. 4. Nuclear fission completed; nuclear parts divided and lying in contact. X 1,012. 



Fig. 5. Direct unilateral nuclear fi.ssion; initial stage. Xl,012. 



Fig. 6. Direct unilateral nuclear fission ; final stage; cell of connective-tissue type; nuclear parts connected only by 

 the merest filament; centrosphere between nuclear sacs; mitochondria streaming across the narrow- 

 connecting strand. Drawn from preparation No. 2 (Lewis); 7-day chick heart grown for 5 days in 

 Locke (1 per cent dextrose); osmic-acid vapor and iron hematoxylin. X 1,0.32. 



Fig. 7. Nuclear fission completed; growth from heart membrane; cell similar to that shown in figure 4, but prepared 

 to show the centrosphere and mitochondria; the single centrosi)here contains two centrosomes; its 

 position, opposite the line of contact of the two nuclear portions (below and to the right), is characteristic. 

 No. 42 (Lewis) (see fig. 1). Xl,032. 



Pl.\te n. 



Fig. 8. Final stage of direct bilateral nuclear fission in a cell of connective-tissue type; shows the somewhat unequal 

 nuclear parts joined by a very slender thread, apparently the attenuated nuclear membrane; overlying 

 this are several strands of mitochondria, a similar relationship to that of figure 6; the larger nuclear sac 

 contains two nucleoli; the smaller but one. .\ large centrosiihere, from which many mitochondria 

 radiate, is conspicuous. The entire cell is very thin and shows mitochondria streaming out into the 

 ])roces.ses. The morjjhology and arrangement of the mitochondria is characteristic for the connective- 

 tissue cell growing in vitro, at periods other than mitosis. No. 17, 24 : 1 1 : 14 (Lewis). 6-day chick 

 stomach; Locke (I per cent dextrose); 3-daygrowth; osmic-acid vapor and iron hematoxyhn. Xl,012. 



Fig. 9. A binueleate cell from heart membiane; the two parts are somewhat separated, and lying between them a 

 single centrosphi're and-mitochondria are to be seen; the latter resemble cocci or short bacilli and show 

 the characteristic radia arrangement about the centrosphere. This tj'pe of mitochomlria is found in 

 cells of membranes growing from chick hearts. No. 42 (Lewis) (.see fig. 1). X 1,012. 



Fig. 10. Nucleus of distorted form in cell of connective-tissue type found in a culture growing in a weak alcoholic 

 medium. The nucleoli in this preparation show as aggregations of granules; this appearance of the 

 nucleoli in connecti\e-ti.ssuc cells stained in this way is found when difTerentiation with iron alum is 

 carried too far. Mitochondria are apparently uninjured. No. 23, 24: 11 : 14 (Lewis). 6-day chick 

 stomach grown in Locke (1 per cent dextrose) to which ethyl alcohol had been added to make approxi- 

 mately 1 per cent; 3-day culture; osmic-acid vapor and iron hematoxyhn. X 1,012. 



Figs. 11 , 12, 13. These figures show a simi^le degree of nuclear fragmentation. They were foun<l in a culture in which 

 the cells were otherwise apparently normal. In figure 11 the nucleus is constricted at one end; the 

 larger portion contains two nucleoli of imequal size and irregular shape. In figures 12 and 13 the con- 

 striction is farther advanced. No. 14, Lewis (see fig. 2). Xl,012. 



Fig. 14. Prophase of mitosis. Nuclear membrane and nucleoli are disappearing and skein is forming; cell not yet 

 rounded. No. 14, Lewis (.see fig. 2). This, and the foiu' figures which follow it, reprcM-nl the process of 

 mitosis in a m()nonu<loate cell. .\ll drawn from the .same preparation. Xl,012.. 



Fig. 15. Metaphase. Cell rounded and compact; processes drawn in; cytoplasm granular and stains verj' dcn.sely 

 with hematoxylin; definite spindle with equatorial plate of chromosomes. Xl,012. 



Fig. 10. Anaphase. The chromosomes ha\e separated and the remains of the spindle may be seen as faintly define(l 

 streaks connecting the two aggregations of chromo.somes; cytoplasm still dcn.sely granular and darkly 

 staining, the entire cell contracted; cell-processes small and thread-like. X 1,012. 



Fig. 17. Early telophase. Chromosomes less clearly marked, the chromatin masses breaking up. No evidence of 

 nuclear membranes is to be seen. The cytoplasm is dividing, as shown by constriction about the 

 eqiiator. Markedly granular and darkly staining protoplasm. X 1,012. 



105 



