Adam Clarke on Exodus 12:9

With the purtenance thereof - All the intestines, for these were abused by the heathens to purposes of divination; and when roasted in the manner here directed they could not be thus used. The command also implies that the lamb was to be roasted whole; neither the head or legs were to be separated, nor the intestines removed. I suppose that these last simply included the heart, lungs, liver, kidneys, etc., and not the intestinal canal.