بارك الله فيكم جميعا على هذا الموضوع الطيب الشيق ولي اضافة بسيطة في هذا الموضوع
خاص عدد من علماء الغرب عددا من التجارب والأبحاث حول عجب الذنب وكان من بين هؤلاء العلماء العالم الألماني Hans Spemann حيث بدأ بدراسة عجب الذنب لدى البرمائيات منذ 1931 و في تجاربه قام بزراعة المخلق الأولي كما أسماه في جنين فنما جنين جنين ثانوي ثم قام بسحق عجب الذنب و عمل نفس التجربة فنما جنين ثانوي، وقام أيضا بغليه فخرج له جنين ثانوي آخر.. وقد نال هذا الدكتور جائزة نوبل على اكتشافه لل PRIMITIVE STREAK سنة 1935 للميلاد .
للمزيد من المعرفة حول هذا العالم وما قام به يمكنكم الدخول عبر هذا الرابط :-
وهذا هو أحد أبحاثه المشهورة في هذا المجال تحت عنوان انتاج الأجنة والتواءم :-
The Production of Twin Embryos and of Duplications in Urodeles by Constriction
Spemann
Hans Spemann was born on June 27, 1869, in Stuttgart Germany. German embryologist who was awarded the Nobel Prize for Physiology or Medicine in 1935 for his discovery of the effect now known as embryonic induction, the influence exercised by various parts of the embryo that directs the development of groups of cells into particular tissues and organs.
Spemann, initially a medical student, attended the universities of Heidelberg, Munich, and Wurzburg and graduated in zoology, botany, and physics. He worked at the Zoological Institute of Wurzburg (1894-1908), held a professorship at Rostock (1908-14), was director of the Kaiser Wilhelm Institute for Biology in Berlin (1914-19), and occupied the chair of zoology at Freiburg (1919-35).
Spemann's concept of induction was based upon a lifetime of research into the early development of the newt. His work showed that, in the earliest stages, the fate of the embryonic parts has not been determined: if a piece of presumptive skin tissue is excised and transplanted into an area of presumptive nervous tissue, it will form nervous tissue, not skin. These results illuminated not only normal processes of development but also the origin of congenital abnormalities. Spemann summarized his researches in Experimentelle Beitrage zu einer Theorie der Entwicklung (1936; Embryonic Development and Induction).
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The Production of Twin Embryos and of
<CENTER>Duplications in Urodeles by Constriction</CENTER>
In the urodele, twin embryos and duplications can be obtained best by constricting the Rap2- cell stage in the plane of first cleavage, using a baby’s hair. When the 2 blastomeres were separated completely by deep constriction, the following results were obtained
a. In certain percentage of the cases, whole embryos developed. They were identical twins. They were smaller in size than normal, but otherwise normal.
b. In a larger percentage, only one blastomere gave a normal embryo, whereas the other half remained unorganized, though viable, round structure.
> In the first case, the cleavage plane coincided with the future median plan of the embryo. In the second case, the first cleavage plane separated future dorsal from future ventral structures, and only the dorsal half gave a normal embryo. The sphere with no axial organs was called family storythe "belly piece" (Bauchstuck). This implies that, in the salamander, the first cleavage plane and the median plane of the embryo have no constant relation. In some eggs, both isolated blastomeres shared in the blastopore and underwent gastrulation, and these eggs invariably gave rise to identical twins. In the majority of cases, only one half egg formed a dorsal lip and gastrulated, and it was this half that developed into the complete embryo, the other forming a belly piece. Spemann concluded that as early as the 2 cell stage, the dorsal half differs from the ventral half in that it contains some property which enables it to undergo typical gastrulation and differentiation. This "dorsal quality", which is lacking in the ventral half, was identified later as the "organizer", and the constriction experiment may be considered the first step in this discovery. 
Procedure:
1. Prepare the hair for constriction, make a loop that is slightly larger than the smaller diameter of the oval capsule.
2. Prepare the embryo – Observe the eggs, waiting for the first cleavage that occurs about 8 – 10 hours after fertilization.
3.Constriction – Wait until the 2 – cell stage is well under way. Prepare the constriction as shown in the following diagram
In order to obtain complete twins, it is not necessary to constrict until the two halves are entirely separated. Such a deep constriction usually results in the bursting of the vitelline membrane and the loss of one per both halves. In order to obtain conjoined anterior duplications, constrict only slightly.
5. After constriction is finished, cut off the free ends of the loop near the egg.
6. Place egg in a clean dish with a dilute medium.
7. Observe gastrulation. It is indicative of the future result. Twins will be obtained if the blastopore and upper lip are shared by both sides. If gastrulation in only one half, then you can conclude that the plane of constriction was frontal. The half that does not gastrulate will form a belly piece.
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