Materials and Methods

Abstract

The grass snake Natrix natrix L. is the most common snake species in Poland and Europe. Individuals do not show distinct dimorphic differences which would make identification of males and females easy and accurate, especially in young and medium -length specimens. Dobrowolska (1990) has reported two mating seasons in this species: early spring and summer. It also seems possible that there is only one prolonged mating season beginning in early spring and finishing in late autumn (Frommhold 1959). When late mating occurs, fertilized females hibernate and lay eggs after emerging in spring (Juszczyk 1987). My personal observations seem to confirm such a possibility. A single female produces and lays eggs in numbers ranging from 9 to 28 (Juszczyk 1987), 6 to 20, and exceptionally, when the eggs are laid immediately after the winter hibernation, their number decreases to 2-6 (personal observation). However, in the latter case, the eggs are usually unfertilized or the embryos do not survive. Mature eggs are elongated or cylindrical in shape and white or cream in color. Eggs are laid by a female in an uninterrupted sequence or almost simultaneously so they adhere quite firmly to one another and form differently shaped, irregular, bunchlike or string-like aggregations. The eggs are laid at sites where appropriate conditions of high humidity and a temperature of 25-30°C prevail. In laboratory conditions, females laid eggs in moist moss or peat. It most often took place at dawn or very early in the morning (personal observation). The eggs were immediately transferred to incubators. The reports in the pertinent literature, as well as my personal observations, confirm that under unfavorable conditions females may refrain from egg laying until conditions improve. Such instances were often observed during winter or an exceptionally dry summer. Grass snake embryos undergo first developmental stages within eggs while still inside the oviduct of a female. In laboratory conditions, the embryos have already completed primary organogenesis when the eggs are laid. The length of the egg incubation period depends on the developmental stage of the embryos at egg laying and on the environmental temperature. My personal observations indicate that the incubation period for grass snake eggs in laboratory conditions at 30°C and 100% relative humidity ranges from 30 to 33 days. The total length of the development of the embryos in this snake species (from fertilization to hatching) has not been examined in laboratory conditions so far.