Figures 339-342. — Squamous epithelial sheath cells covering down feathers. These cells separate 

 readily and as a contaminant frequently fall on impression smears made from embryonic and young 

 chick organs. 2,470 X. 



339 A relatively small cornified down-sheath cell found 

 in bone-marrow smear. Oval area of dead nucleus 

 slightly to the left of center. No keratin granules. 

 Chick just hatched. 



340 An elongated down-sheath cell found in bone-marrow 

 smear. Dead nucleus in the center. Numerous 

 keratin bodies formed in the cytoplasm. Chick just 

 hatched. 



341 A large and slightly smudged down-sheath cell found 

 in a spleen smear. Coalescence of keratin bodies. 

 No nucleus visible. Orange body with a dense 

 nucleus is a broken erythrocyte on the surface of the 

 sheath cell. Embryo incubated 20 days 4 hours. 



342 A final stage in the keratinization of a down-sheath 

 cell found in bone-marrow smear. Remains of the 

 dead nucleus visible in the center. Embryo incu- 

 bated 19 days 19 hours. 



176 



