May the physician call in a physician who, he knows, will not scruple to perform the therapeutic abortion on an unviable fetus?
By foetus immaturus here he means an unviable foetus, as is evident from the context.
viable vi·a·ble (vī'ə-bəl)
Capable of living, developing, or germinating under favorable conditions.
Capable of living outside the uterus. Used of a fetus or newborn.