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The membranes appear as thin, white delicate structures either organized in a circular pattern or as irregularly shaped masses of tissue. The size of the memebranes varies significantly, ranging from a 1 cm diameter to encompassing the entire surface of the yolk. In some instances, removing the egg yolk was necessary to detect the presence of membranes localized on the bottom of the yolk.
The above image is that of an unfertilized egg (the arrow identifies the location of the blastodisc) following 8 days of incubation. The following images are examples of the extensive variability in the expression of unorganized membranes observed in parthenogenetic eggs incubated between 4 and 21 days.
Ruptured Vitelline Membrane (Yolky)
This category is characterized by a whitish-yellow color of the egg's contents. Membranous tissues may be present but due to the destruction of the vitelline membrane, the yolk's contents are mixed with albumen. This condition is similar to that observed in fertilized eggs whose embryonic development ceased following 3 to 5 days of incubation. Differentiation of this parthenogenetic expression, from a post-fertilization incubation failure is difficult and may not be conclusive.The following five images identified as Ruptured Membranes illustrate this category of parthenogenetic expression that was observed between 7 and 21 days of incubation.
Typically , parthenotes of this category are characterized by the presence of small aggregates of embryonic blood cells. The vitelline membrane may have ruptured. The appearance of the blood cells present varies depending on the stage of embryonic development. The blood cells may appear as small irregular dots dispersed on membranous tissue or may be organized in a pattern referred to as a blood ring. Typically, this form of expression with visible blood cells is not as commonly observed as are membranes or the yolky, the three images that follow this narrative are examples of parthenogenesis with blood in eggs incubated for 7 to 10 days.
The most infrequently observed form of parthenogenesis is the developing embryo. Typically, a parthenogen of this category is characterized by delayed embryonic development of 2 to 3 days when compared with the expected development for an embryo incubated for a given period of incubation hours. Typically, parthenogenetic embryos die in ovo but those that are able to sustain a modem of normal development require an additional 2 to 3 days to hatch. There is documentation that parthenogenetic poults hatching were only males (Olsen, 1957). The following two images are developing parthenogenetic embryos observed at 10 and 14 days, respectively.
Attempts to discriminate between parthenogenetic development and early embryonic failures prior to the formation of blood cells may result in mis-classifications. Chart 1 summarizes a study in which 1510 eggs from Medium White Wrolstad virgin hens were artificially incubated for varying periods of time from 48 hours to 21 days. The largest incidence of parthenogenesis was observed in eggs incubated for 12 days. This data suggests that to perform a practical hatchery breakout for the macroscopic identification of parthenogenetic embryos, the incubation period should be at least six days.
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Categories of Parthenogenetic Expression
Proceed to General Discussion of Parthenogenetic Expression