The membranous labyrinth of the opossum Didelphis virginiana is still in a relatively early stage of development at birth and is entirely nonfunctional. This opossum is born thirteen days after fertilization and completes its development in the brood pouch of the mother. At the time it is born most of its organs are very embryonic, but by fifty days post partum it has reached a stage of development approximating that of most newborn mammals (McCrady1). The opossum in the pouch, therefore, provides embryonic material easy of access for experimental purposes.
Langworthy2 noted the earliest sign of vestibular response in pouch young at 41 days, while Larsell, McCrady and Zimmermann3 reported such responses at about 43 days. Acoustic function does not appear until about a week later. Larsell and McCrady4 and Larsell, McCrady and Zimmermann reported acoustic responses in the form of a spasmodic contraction of trunk musculature in pouch young
LARSELL O, MCCRADY E, LARSELL JF. DEVELOPMENT OF THE ORGAN OF CORTI IN RELATION TO THE INCEPTION OF HEARING. Arch Otolaryngol. 1944;40(4):233–248. doi:10.1001/archotol.1944.00680020309001
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