Postmarked the Stars by Andre Norton

Trouble started early for the free-trader Solar Queen the moment cargo master Dane Thorson collected a package for transit from Xecho to Trewsworld. What was in the package Dane did not learn until after he had been kidnapped, knocked out, and a dead man bearing his credentials had been substituted for him aboard the ship.

Efforts to find out the reasons for the mysterious switch seemed hopeless until they made the ghastly discovery that their precious cargo of alien embryos had been subjected to secret radiation, causing them to regress genetically into monster forms.

Only after Dane and his fellow spacemen had endured a series of nightmare adventures on their port of call did they begin to fit the puzzle together--to realize that they had stumbled upon a criminal operation that involved a whole cluster of stars.

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