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...
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