iiulicaled the i^tale of knowledge about this ceil 

 for a considerable length of time. 



Cells like figure 342 were most commonly 

 seen. They occurred in most bone-marrow 

 smears and later in some spleen smears. Since 

 all the cells in figures 339-342 were drawn at 

 the magnification used for other high-power 

 drawings, it is apparent that they were many 

 times larger than erythrocytes. When cells like 

 figure 340 were found, a protozoan parasite was 

 suspected, but none were known that closely ap- 

 proached this morphology and retained a large 

 central nucleus. As different tissues were used 

 for smears, the more widely did these cells seem 

 to be distributed. Finally, after they had lieen 

 seen in smears from connective tissue of the groin 

 region and from air sacs as well as from bone 

 marrow, thymus, spleen, and bursa, it became 

 evident that this was a contaminant. 



A search for the source led to an examination 

 of the surface of the late embryo and of the young 

 chick, the scalps of the people who were making 

 the slides, and the towels used to wipe the hands 

 and instruments. 



The x-cells were obtained only from the sur- 

 face of the chick. If a slide was covered with 

 albumen and the down was brushed with a pencil 

 while the newly hatched chick was held several 

 inches above the slide, many cells of the type 

 shown in figures 339-342 were seen. It was 

 then but a short step to locate the sheath of cells 

 that encloses the emerging down. When they 

 were seen in sheets, as in 343 B, it was evident 

 whence they had come. The remarkable thing 

 about them was their ability to separate them- 

 selves so easily and completely from other cells 

 of the same epithelium that nearly always they 

 were found as isolated cells. Although their 

 source was now known, there was still the ques- 

 tion of how they came to be a contaminant on 

 impression smears, since the down of many em- 

 bryos was wet and it was unlikely that the cells 

 could be spread by air. The crevices of the 

 mouse-tooth forceps used to ])ull the tissues apart 

 carried these cells. Although the forceps were 

 not touched to the slide when the smear was 

 made, they must have been the chief source lie- 

 cause, subsequent to this period of investigation, 

 no down-sheath cells appeared in smears if the 

 teeth of the forceps were carefully cleaned after 

 each step in the dissection process. 



After it had been learned that these cells 

 were specifically associated with the down of the 

 young chick or the late embryo and that they were 

 readily separated and disseminated, it was de- 

 cided to conduct an experiment with two of the 

 men wlio take off the chick hatch each week. 

 This Laboratory maintains a quarantine, and a 

 man coming onto the premises changes his 

 clothes, washes his hands, and puts on boots and 

 coveralls furnished by the Laboratory. When 

 men enter the incubator rooms, a second change 

 of clothing is made. After the two men had 

 taken off the hatch they went through the quaran- 

 tine in reverse, so that they entered the Labora- 

 tory in their street clothing. Previously 6 al- 

 buminized slides had been prepared. Now each 

 man was asked to brush his hair about 6 inches 

 from 3 of the slides. 



All 3 slides of 1 group showed sheath cells 

 when stained, and 1 of the other 3 slides showed 

 them. Therefore, any truly effective quarantine 

 should eliminate the possible transfer of these 

 cells from one group of chicks to another, or else 

 it should be shown that these desquamated cells 

 cannot serve as carriers of pathogenic organisms. 



The cells selected for illustration are intended 

 to show three steps in the process of keratiniza- 

 tion (figs. 339, 340, and 342) and one atypical 

 cell (fig. 341). In figure 339 a nuclear ghost is 

 indicated by its slight orange color and oval shape. 

 In this cell the cytosome is uniforndy vacuolated 

 and the ground substance takes a light-blue stain. 

 Often the tliin edges of these cells are folded and 

 in this case there are 2 folds, 1 above and 1 be- 

 low the level of the nucleus. 



Usually the cells have greater length than 

 width. The nucleus is orientated with its long 

 axis transversely placed across the middle of the 

 cell (fig. 340). It, of course, is dead and no 

 chromatin particles are visible, only a faint 

 orange coloration without structure. Keratin 

 particles develojj in the cytosome from many 

 separate centers. Some grow larger and flow 

 together with adjoining masses. The typical ap- 

 pearance of the last stage when no more keratin- 

 ized material can be packed into the cell is 

 shown in figure 342. The nuclear ghost is visiljle 

 ill the center of the cell. The cytosome is divided 

 into multisided angular compartments with a 

 small residue of Idue-staining cytoplasm between 

 them to mark their boundaries. The intense 



179 



