Chelonian eggs may be leathery or parchment-shelled that indent easily when laid and expand due to water uptake during incubation; harder-shelled but expansible eggs which behave much like parchment eggs during incubation; and brittle-shelled eggs which do not indent when laid and usually take up little or no external water during incubation (Ewert, 1979, 1985; Kuchling, 1999).
Source: wiktionary