This venom is derived from b-defensins, proteins that are present in mammals that create holes in viral and bacterial pathogens. Some reptile venom is also composed of different types of b-defensins, another trait shared with reptiles. It is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs. The key anatomical difference between monotremes and other mammals gives them their name; "monotreme" means “single opening” in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Monotremes, like reptiles, have a single cloaca; marsupials also have a separate genital tract; whereas most placental mammal females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only semen passes through the penis; urine is excreted through the cloaca. The monotreme penis is similar to that of turtles, and is covered by a preputial sac. Monotreme eggs are retained for some time within the mother and receive nutrients directly from her, and they generally hatch within 10 days after laying, much shorter than the incubation period of sauropsid eggs. Newborn monotremes, called "puggles", are larval and fetus-like, much like newborn marsupials (and perhaps all non-placental mammals), and like them have relatively well-developed forelimbs that enable them to crawl around. In fact, because monotremes lack nipples, their puggles crawl about more frequently than marsupial joeys in search of milk; this difference raises questions about the supposed developmental restrictions on marsupial forelimbs. Monotremes lactate from their mammary glands via openings in their skin, rather than through nipples. All five extant species show prolonged parental care of infants, with low rates of reproduction and relatively long life-spans. Monotremes are also noteworthy in their zygotic development: Most mammal zygotes go through holoblastic cleavage, meaning that after fertilization, the ovum splits into multiple, divisible daughter cells. In contrast, the zygotes of monotremes, like those of birds and reptiles, undergo meroblastic (partial) division. This means the cells at the yolk's edge have cytoplasm continuous with that of the egg, which allows the yolk and embryo to exchange waste and nutrients with the surrounding cytoplasm.