Wilkin on Vipers
A note to Pseudodoxia Epidemica, Book III, chapter 16

This [i.e., that the mother "upon a fright" swallows her young] is admitted to be true of the rattle-snake, but denied of the viper. I subjoin two passages from Cuvier, by Griffiths, vol.. ix, pp. 344, 356.

"The crotali are viviparous; at Martinique it is the general persuasion that the offspring are eaten by the vipers when they are very young, and a little after their birth. According to M. Palisot de Beauvois, this prejudice derives its origin from a fact wrongly interpreted. In the first journey made by this naturalist, in the country of the native Teharlokee, he saw a crotalus horridus in a path, and approached it as softly as possible. At the moment when it was about to be struck, the animal agitated its rattles, opened a wide throat, and received into it five little ones, about as thick each as a goose-quill. But at the end of ten minutes, believing itself out of danger, it opened its mouth and let the young ones out, which, however, entered there again, on the appearance of a new danger. M. Guillemart, a countryman of our own, has verified the same fact."

"In the fine days of early spring, the vipers may be seen basking in the morning sun, on little hills exposed to an eastern aspect, and they speedily occupy themselves in the great work of propagating the species. The act of generation takes a very long time in its accomplishment, and its result is the vivification of from twelve to twenty-five eggs, almost as large as those of wrens or titmice. These exclude the young, in the womb of the mother, and there they remain coiled up, and come to the length of three or four inches before they issue forth, which they generall do in the course of the fourth month after fecundation. Having thus, by a sort of parturition, quitted their mother, the young vipers, for some time after, carry with them the remains of the egg which enclosed them, and which then have the appearance of irregularly torn membranes. But from that time they are entirely strangers to the being which gave them birth, and do not seek refuse in her mouth, on the approach of danger, as the ancients erroneously imagined."

This resemblance of the remains of the egg which the young vipers carry about with them, to "irregularly torn membranes," may possibly have promoted the popular error under discussion. White has the following remarks:

"Though they are oviparous, yet they are viviparous also, hatching their young within their bellies, and then bringing them forth. Whereas snakes lay chains of eggs every summer in my melon beds, in spite of all that my people can do to prevent them; which eggs do not hatch till the spring following, as I have often experienced. Several intelligent folks assure me that they have seen the viper open her mouth and admit her helpless young down her throat on sudden surprises, just as the female opossum does her brood into the pouch under her belly, upon the like emergencies; and yet the London viper catchers insist on it, to Mr. Barrington, that no such thing every happens."

"On August the 4th, 1775, we surprised a large viper, which seemed very heavy and bloated, as it lay in the grass basking in the sun. When we came to cut it up, we found that the abdomen was crowded with young, fifteen in number; the shortest of which measured full seven inches, and were about the size of full-grown earth-worms. This little fry issued into the world with the true viper-spirit about them, showing great alertness as soon as disengaged from the belly of the dam: they twisted and wriggled about, and set themselves up, and gaped very wide when touched with a stick, showing manifest tokens of menace and defiance, though as yet they had no manner of fangs that we could find, even with the help of our glasses."

"There was little room to suppose that this brood had ever been in the open air before; and that they were taken in for refuge, at the mouth of the dam, when she perceived that danger was approaching; because then probably we should have found them somewhere in the neck, and not in the abdomen."