Plate 18 



A. Section of gizzard of 17-day-old chicken embryo showing threads of 

 keratin S passing down the lumen L of the secreting epithelium. The 

 secretion gives a positive test for disulphide groups and appears to form 

 by the coalescence of fine filaments f. 



B. Cross-section through an egg-shell membrane of the chicken egg. In 

 the light microscope the membrane is seen to consist of fibrils roughly 

 hexagonally arranged parallel to the egg surface. In e.m.gs. the fibrils are 

 cut in various directions and each is seen to consist of two components 

 (1 and 2); the inner component has reacted more strongly than the outer 

 with the osmium fixative and may thus be the keratin component (see 

 p. 107). 



C. Part of a secreting cell. 



