The egg-shell membranes of the chicken, duck, and turkey contain 0.66 to 1% hydroxyproline depending upon the sp. The remainder of the contents of the unincubated egg contains no detectable amt. of hydroxyproline. Hydroxyproline appears in detectable amts. in the chick embryo after the 4th day and increases to 1.16% of the dry wt. on the 19th day. The serosa contains hydroxyproline after the 5th day which increases to a max. of 2.1% on the 14th day. Most. of the hydroxyproline of the embryo and serosa is present in a form extractable by autoclaving, probably as collagen.