Likutei Moharan, Part II 85:2/ליקוטי מוהר"ן·תנינא פ״ה:ב׳

12. And this is the Three Weeks, namely the twenty-one days of Between the Straits. Among trees, the counterpart to this is the luz. This is the reason it is customary to eat an egg for the meal before the fast of Tisha b’Av. For an egg matures in twenty-one days, as our Sages, of blessed memory, taught: A hen lays her egg after twenty-one days. Among trees, the counterpart to this is the luz (Bekhorot 8a). This is also the luz bone at the back of a person’s neck. It will remain after the body decays, and from it the body will be built anew at the time of Resurrection of the Dead.

2And that is our main consolation, the concept of “Nachamu, nachamu,” as mentioned above. For about the body it is stated “lie down until morning” (Ruth 3:13). This refers to the time of the Resurrection. “And when the morning came, look, she is Leah!” (Genesis 29:25)—i.e., the aforementioned luz bone through which the body is rebuilt.