ANIMALS: Half-Baked Babies

Far down the evolutionary scale of mammals are the marsupials (pouch-bearers). The Western Hemisphere’s only pouch-bearer, the opossum, is the lowest marsupial of all. Its young, born after a gestation of about 13 days, are only one-half inch long at birth, without fully-developed hind legs, sans eyes, sans ears or reflexes. A litter of 18 weighs about 1/15 oz., fits easily into an ordinary teaspoon (see cut). Because the opossum is born at such an early stage in its development it makes glad the hearts of embryologists.

Few years ago Philadelphia’s Wistar Institute of Anatomy and Biology managed to raise a few opossums in captivity. This year 285 young were born on its farm. From his study of these. Edward McCrady Jr. last week published a monograph on The Embryology of the Opossum, which brought to an end one controversy, may well start a few more.

Long accepted by embryologists has been a theory that the marsupial mother tucks her newborn into the pouch with her lips. When, at various times, scientists reported cases of kangaroo and opossum babies crawling up their mothers’ inclined bodies into the incubating pouches by themselves, most of their colleagues raised an eyebrow. The fetuses, they said, were too immature for such mountaineering.

But according to Dr. McCrady, the young opossums do exactly that: after the mother has moistened the hair on her abdomen, they slowly pull themselves by the claws on their forefeet up the incline into the soft, warm, apron-pocket pouch. The mother sits quietly on her haunches, takes no part in the affair. It is likely that many of the young, with little but instinct to guide them, miss the mother’s pouch entirely. The number found there is al most always less than the number of embryos in the uterus shortly before birth.

In the pouch the youngsters stay for about 55 days, attached to their mother’s body by the swelling of her nipples, too big for their tiny mouths to release. When they have developed to the size of a mouse they crawl about her body, with frequent visits home. Thirty days later, fully developed in everything but size, they leave home for good; but not until three years have passed do they reproduce their teaspoonfilling kind.