Dr. Muhammad `Ali Al Barr referred that the Prophet (may the peace and the blessings of Allah be upon him) has pointed out that no part of human body remains safe except the coccyx bone and when Allah will intend to resurrect the bodies on the Day of Resurrection he will send down from the sky a rain like the sperms of men then people would spring out from the remnants of the primitive node inherent in the tailbone or sacrococcygeal region. (*)
Perhaps, this statement refers to the Hadith narrated on the authority of Abu Hurayrah (may Allah be pleased with him) that the Prophet (may the peace and the blessings of Allah be upon him) said: “Between the two sounds of the trumpet, there will be forty." Somebody asked Abu Huryrah, "Forty days?" But he refused to reply. (1)Then he asked, "Forty months?" He refused to reply. Then he asked, "Forty years?" Again, he refused to reply. Abu Huryrah added. "Then (after this period) Allah will send water from the sky and then the dead bodies will grow like vegetation grows." And the Prophet (may the peace and the blessings of Allah be upon him) said: “There is nothing of the human body that does not decay except one bone; that is the little bone at the end of the coccyx of which the human body will be recreated on the Day of Resurrection."
[Reported by Al Bukhari (3) and Muslim (4) but the words of this version belong to Muslim]
Another version of this Hadith includes “The earth would consume every part of the son of Adam (after his death) except for one bone; the coccyx (the bone at the base of the vertebral column) from which he was created and from which his body will be reconstituted on the Day of Resurrection.'' [Reported by Muslim] (5).
Another version of the Muslim’s narration says: “There is a bone in the human being whom the earth would never consume; from this the new bodies would be reconstituted on the Day of Resurrection. They asked: O Allah's Messenger! Which bone is that? Then he said: It is the coccyx bone.”
Dr. Muhammad `Ali Al Barr hold this Hadith as evidence in his book (Deformed Foetus and Genetic Diseases) (6), after talking about the formation of the nervous system, he referred to this Hadith. However, the point of his reasoning is not clear to me, so I am quoting some part of his explanation about the primitive node or primitive streak of human body, he says in this context: "The first phase of the nervous system begins to appear after the formation of primitive node which appears since the fifteenth day of conception."
Then he adds: "Since the appearance of the first node and its extensive activity, the following developments take place:
1 – The appearance of notochord (or dorsal nerve cord, or primitive spinal column) that extends towards the head from the primary node or primitive steak.
2– After the initial appearance of the node, the embryonic disk turns into the shape of a pear so both ends could be distinguished; the wider end is called “Head Side” while the thin end is called the “Caudal Side” or the tail side.
3 - Then the nervous system starts to appear...” Furthermore, Dr. Muhammad `Ali Al Barr continued his explanation under the title of (Fate of the First Node) and added saying: "The first node -as we have said- is very important; because its extensive activities lead to the formation of notochord (or the spinal column) and forms the interior middle layer or what is called (the Mesoderm) which condenses on the side of the axis forming the fetus. Whenever the first node completes its mission during the fourth week it begins to emaciate and remains hidden in the sacrococcygeal region of the foetus, then in the baby and afterwards it disappears totally except its small remnant part which could not be seen with the naked eye. And the Prophet (may the peace and the blessings of Allah be upon him) also pointed out that nothing of human body remains safe except the coccyx bone."
(*)The Deformed Foetus and Genetic Diseases, P. (403).
(1) An-Nawawi said: “Abu Hurayrah(may Allah be pleased with him) intends that he was un-confident whether it was forty days or months or years, but he was confident that it was forty in general." See Commentary of An-Nawawi to Sahih of Muslim (18/91, 92).
(2) The Arabic word "Al `Ajbu” according to Ibn Al Athir, is the bone at end of the spinal column. See An-Nihayah (3/184). An-Nawawi said: "It is the delicate bone at the end of the spinal column the head of tailbone; it is also pronounced as “Al `ujmu”. See, Commentary of An-Nawawi to Sahih of Muslim (18/92).
Ibn Al Faris said: "It is the meeting point of every animal’s both hips in the end of the backbone at the coccygeal zone." See, the Arabic Dictionary “Maqayees Al lughah” (4/244). In short, “`Ajm Adh-Dhanab” or “`Ajb Adh-Dhanab” is a small and tiny bone at the end of the backbone, at the edge of the human and animal vertebral column.
(3) Sahih Al Bukhari -The Book of Commentary (Kitab Al-Tafsir), Chapter: “ And the Trumpet will be blown, and all who are in the heavens and all who are on the earth will swoon away, except him whom Allâh wills. Then it will be blown a second time, and behold they will be standing, looking on (waiting) (Az-Zumar:68)” (8/551, 552 Hadith No.4814) and the Chapter: “The Day when the Trumpet will be blown, and you shall come forth in crowds (groups after groups).” [An-Naba’: 18] (8/689,690,Hadith No.4935).
(4) Sahih Muslim - The Book Pertaining to the Turmoil and Portents of the Last Hour (Kitab Al-Fitan wa Ashrat As-Sa`ah), Chapter: “Whatever would be between the two sounds of the trumpet” (4/2270,2271, Hadith No.4955).
(5) Sahih Muslim,(4/2271, Hadith No.142).
(6)The Deformed Foetus and Genetic Diseases, P. (403).