topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and force it downward. It is common for neck-biting to occur while the snakes are entwined. Facultative parthenogenesis Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization. Agkistrodon contortrix (copperhead) and Agkistrodon piscivorus (cottonmouth) can reproduce by facultative parthenogenesis, meaning that they are capable of switching from a sexual mode of reproduction to an asexual mode. The most likely type of parthenogenesis to occur is automixis with terminal fusion, a process in which two terminal products from the same meiosis fuse to form a diploid zygote. This process leads to genome-wide homozygosity, expression of deleterious recessive alleles, and often to developmental abnormalities. Both captive-born and wild-born copperheads and cottonmouths appear to be capable of this form of parthenogenesis. Reproduction in squamate reptiles is almost exclusively sexual. Males ordinarily have a ZZ pair of sex-determining chromosomes, and females a ZW pair. However, the Colombian Rainbow boa (Epicrates maurus) can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny. The WW females are likely produced by terminal automixis. Embryonic Development Snake embryonic development initially follows similar steps as any vertebrate embryo. The snake embryo begins as a zygote, undergoes rapid cell division, forms a germinal disc, also called a blastodisc, then undergoes gastrulation, neurulation, and organogenesis. Cell division and proliferation continues until an early snake embryo develops and the typical body shape of a snake can be observed. Multiple features differentiate the embryologic development of snakes from other vertebrates, two significant factors being the elongation of the body and the lack of limb development. The elongation in snake body is accompanied by a significant increase in vertebra count (mice have 60 vertebrae, whereas snakes may have over 300). This increase in vertebrae is due to an increase in somites during embryogenesis, leading to an increased number of vertebrae which develop. Somites are formed at the presomitic mesoderm due to a set of oscillatory genes that direct the somitogenesis clock. The snake somitogenesis clock operates at a frequency 4 times that of a mouse (after correction for developmental time), creating more somites, and therefore creating more vertebrae. This difference in clock speed is believed to be caused by differences in Lunatic fringe gene expression, a gene involved in the somitogenesis