Seriously, the first time I saw a picture of a snake fetus, I still didn't believe it. From the impression of childhood, snakes are all oviparous, mammals are fetal, and snakes are cold-blooded animals. And it is impossible to imagine how a fetal snake can breastfeed.
First of all, mammals are not all viviparous, for example, in the biology class, there is a special case mentioned - platypus is egg-born but breastfeeding. Secondly, the snake will not be a mammal, so strictly speaking, the birth of the snake belongs to the pseudo-fetal birth, or the egg fetus, and the small snake that the snake gives birth to first has a transparent film on the outside, which is the egg shell, but it is very transparent and soft.
ovoviviparous
Oviparous birth refers to the breeding mode in which an animal's eggs develop into new individuals in the mother's body before producing the mother's body. However, the nutrients required for development still rely on the yolk stored by the egg itself, and there is no material exchange relationship with the mother, or a breeding method that only exchanges gases with the mother and has little nutritional connection with the mother in the later stages of embryonic development. It is a breeding method between oviparous and viviparous.
Oviparous fetuses improve the development environment of embryos and larvae for animals that use this method of reproduction, so that the survival rate of hatching is more secure than that of oviparous beings. In addition, oviparous birth is a transitional form from oviparous to viviparous in the evolutionary process of animals, providing another living evidence for Darwin's theory of evolution.
viviparous
The process by which an animal's cells develop in the uterus of an animal is called viviparous birth. The nutrients needed for embryonic development can be obtained from the mother until birth. Viviparation and breastfeeding ensure a high survival rate for offspring. Viviparous life provides protection, nutrition and stable constant temperature development conditions for the developing embryos, which can ensure the normal progress of enzyme activities and metabolic activities, and minimize the adverse effects of external environmental conditions on embryonic development. And the amniotic fluid in the womb can reduce the effect of vibration on the fetus.
In fact, egg birth is not a breeding method only for snakes. Some of the common oviparous animals are sharks, peacock fish and large-bellied fish, as well as some venomous snakes (such as pit vipers, sea snakes) and fetal lizards, copper stone dragon lizards, etc. are oviparous animals. This is the result of the long-term adaptation of animals and the breeding mode formed by the long-term adaptation of animals to poor environments. The other part of the cockroach is also oviparous.