The objective of the study was to demonstrate which measurements of the embryo during the gastrulation phase can be used for estimating their degree of development and determine the hatching value of turkey eggs. The study was performed on hatching eggs of heavy white broad-breasted turkeys, from the 16th, 20lh and 24th week of the laying season. The embryo structure was examined on the third day of incubation, at eighthour intervals. The shape and diameter of germinal discs, embryo length, and the width of the vasculosa area, head and tail regions were determined visually, while the number of somite pairs - under a microscope. The shape of germinal discs changed from round to flat, and their diameter increased during incubation. Embryo length, the width of the head region, and the number of somite pairs increased at eight-hour intervals. The tail region width was smaller in the 72nd hour, compared with the 56th and 64th hour of incubation, and showed wide variation. These measurements indicate progress in development and changes in embryo position. Body measurements of embryos in eggs from the 24th week of the laying season were greater, compared with the 16th week. The only exception was the width of the tail region. Worse hatching results were a consequence of a lower egg fertilization rate and increased embryo mortality between the 11th and 24th days of incubation. To estimate the degree of development of the embryo and hatching value of eggs the following measurements can be used: the shape and diameter of the germinal disc, embryo length, and the width of the head region. These embryo features were characterized by lower variation in comparison to the others. Embryonic development in older turkey eggs proceeded rapidly and final hatching results were worse.
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