Answer:
Dr. Van Helsing has to go back to Amsterdam, and he writes to Seward to keep an eye on Lucy. But Van Helsing's letter is delayed because the address is incomplete, and that delay is fatal.
On September 17th, the night a delzoological wolf escaped, Lucy wakes up frightened by a flap in her window and the howling of a wolf. Mrs. Westenra, who had removed the garlic wreath from Lucy's neck, is also frightened by the noise and runs to her daughter's room. Suddenly, the window shatters and the head of a huge wolf appears.
