The Production of Twin Embryos and of
In the urodele, twin embryos and duplications can be obtained best by constricting the 2- 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
> 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 the "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.
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.
